Zinc supplementation for improving pregnancy and infant outcome

Erika Ota; Rintaro Mori; Philippa Middleton; Ruoyan Tobe‐Gai; Kassam Mahomed; Celine Miyazaki; Zulfiqar A Bhutta

doi:10.1002/14651858.CD000230.pub5

Cochrane Database Syst Rev. 2015 Feb; 2015(2): CD000230.

Published online 2015 Feb 2. doi: 10.1002/14651858.CD000230.pub5

PMCID: PMC7043363

PMID: 25927101

Zinc supplementation for improving pregnancy and infant outcome

Monitoring Editor: Erika Ota, Rintaro Mori, Philippa Middleton, Ruoyan Tobe‐Gai, Kassam Mahomed, Celine Miyazaki, Zulfiqar A Bhutta, and Cochrane Pregnancy and Childbirth Group

National Center for Child Health and Development, Department of Health Policy, 2‐10‐1 Okura, Setagaya‐ku, TokyoJapan, 157‐8535

The University of Adelaide, Women's and Children's Research Institute, Women's and Children's Hospital, 72 King William Road, AdelaideSouth AustraliaAustralia, 5006

School of Public Health, Shandong University, No.44 Wen‐Hua‐Xi Road, JinanChina, 250012

Ipswich Hospital, IpswichQueenslandAustralia, 4305

National Research Institute for Child Health and Development, Department of Health Policy, 10‐1, Okura 2 chome, SetagayaTokyoJapan, 157‐8535

Hospital for Sick Children, Center for Global Child Health, TorontoONCanada, M5G A04

Rintaro Mori, Email: moc.liamg@iromoratnir.

Author information Copyright and License information PMC Disclaimer

Abstract

Background

It has been suggested that low serum zinc levels may be associated with suboptimal outcomes of pregnancy such as prolonged labour, atonic postpartum haemorrhage, pregnancy‐induced hypertension, preterm labour and post‐term pregnancies, although many of these associations have not yet been established.

Objectives

To assess the effects of zinc supplementation in pregnancy on maternal, fetal, neonatal and infant outcomes.

Search methods

We searched the Cochrane Pregnancy and Childbirth Group's Trials Register (31 October 2014) and reference lists of retrieved studies.

Selection criteria

Randomised trials of zinc supplementation in pregnancy. We excluded quasi‐randomised controlled trials.

Data collection and analysis

Three review authors applied the study selection criteria, assessed trial quality and extracted data. When necessary, we contacted study authors for additional information. The quality of the evidence was assessed using GRADE.

Main results

We included 21 randomised controlled trials (RCTs) reported in 54 papers involving over 17,000 women and their babies. One trial did not contribute data. Trials were generally at low risk of bias. Zinc supplementation resulted in a small reduction in preterm birth (risk ratio (RR) 0.86, 95% confidence interval (CI) 0.76 to 0.97 in 16 RCTs; 16 trials of 7637 women). This was not accompanied by a similar reduction in numbers of babies with low birthweight (RR 0.93, 95% CI 0.78 to 1.12; 14 trials of 5643 women). No clear differences were seen between the zinc and no zinc groups for any of the other primary maternal or neonatal outcomes, except for induction of labour in a single trial. No differing patterns were evident in the subgroups of women with low versus normal zinc and nutrition levels or in women who complied with their treatment versus those who did not. The GRADE quality of the evidence was moderate for preterm birth, small‐for‐gestational age, and low birthweight, and low for stillbirth or neonatal death and birthweight.

Authors' conclusions

The evidence for a 14% relative reduction in preterm birth for zinc compared with placebo was primarily represented by trials involving women of low income and this has some relevance in areas of high perinatal mortality. There was no convincing evidence that zinc supplementation during pregnancy results in other useful and important benefits. Since the preterm association could well reflect poor nutrition, studies to address ways of improving the overall nutritional status of populations in impoverished areas, rather than focusing on micronutrient and or zinc supplementation in isolation, should be an urgent priority.

Plain language summary

Zinc supplementation for improving pregnancy and infant outcome

Taking zinc during pregnancy helps to slightly reduce preterm births, but does not prevent other problems such as low birthweight babies.

Many women of childbearing age may have mild to moderate zinc deficiency. Low zinc concentrations may cause preterm birth or they may even prolong labour. It is also possible that zinc deficiency may affect infant growth as well. This review of 21 randomised controlled trials, involving over 17,000 women and their babies, found that although zinc supplementation has a small effect on reducing preterm births, it does not help to prevent low birthweight babies compared with not giving zinc supplements before 27 weeks of pregnancy. One trial did not contribute data. The overall risk of bias was unclear in half of the studies. No clear differences were seen for development of pregnancy hypertension or pre‐eclampsia. The 14% relative reduction in preterm birth for zinc compared with placebo was primarily represented by trials of women with low incomes. In some trials all women were also given iron, folate or vitamins or combinations of these. UNICEF is already promoting antenatal use of multiple‐micronutrient supplementation, including zinc, to all pregnant women in developing countries. Finding ways to improve women's overall nutritional status, particularly in low‐income areas, will do more to improve the health of mothers and babies than supplementing pregnant women with zinc alone. In low‐ to middle‐ income countries, addressing anaemia and infections, such as malaria and hookworm, is also necessary.

Summary of findings

Background

Description of the condition

The overall nutritional status of the mother during pregnancy is a significant contributor to both maternal and perinatal mortality and morbidity (Koblinsky 1995). This is likely to be even more crucial in developing countries where anaemia and infections, such as malaria and hookworm, compound the issue even further.

Description of the intervention

Zinc is known to play an important role in many biological functions, including protein synthesis and nucleic acid metabolism (Valee 1993). Although severe zinc deficiency is now considered rare, mild to moderate deficiency may be relatively common throughout the world (Sanstead 1991). In a review of literature published between 1970 and 1991, Parr 1996 noted that, on average, pregnant and lactating women worldwide consumed 9.6 mg zinc per day, well below the recommended 15 mg daily, during the last two trimesters of pregnancy (Sanstead 1996; WHO 1996). In animal studies, zinc deficiency during the early stages of pregnancy is associated with reduced fertility (Apgar 1970), fetal neurological malformations and growth retardation (McKenzie 1975), and deficiency in later stages of pregnancy negatively affects neuronal growth and may also be associated with impaired brain function and behavioural abnormalities (Golub 1995).

How the intervention might work

In humans, pregnant women with acrodermatitis enteropathica (an inherited defect in zinc absorption from the bowel) show association with increased risk of congenital malformations and pregnancy losses (Verburg 1974). Numerous reports have noted low serum zinc levels to be linked with abnormalities of labour such as prolonged labour and atonic postpartum haemorrhage (Prema 1980), pregnancy‐induced hypertension (Jameson 1976; Jameson 1993), preterm labour (Jones 1981) and post‐term pregnancies (Simmer 1985). Others (Cherry 1981; Chesters 1982) have failed to show any such association.

Some researchers have also reported an association between low zinc and small‐for‐gestational age babies, and poor perinatal outcome (Kiilholma 1984a; Kiilholma 1984b). Kirksey 1994 reported low maternal serum zinc levels during pregnancy to be associated with an increased risk of low birthweight and preterm birth. Low birthweight babies have higher rates of morbidity and mortality due to infectious disease and impaired immunity and, thus, it is possible that zinc deficiency may affect infant growth and well being too.

Why it is important to do this review

Studies of the effects of zinc supplementation have differed in their findings. These inconsistencies in study findings could be due to lack of consensus on accurate assessment of zinc status (Aggett 1991) and to differences in the populations studied. Randomised controlled trials of zinc supplementation in pregnancy would help to address the association, if any, between zinc deficiency and pregnancy outcome and neonatal and infant health and well being.

The fetal nervous system also develops progressively during pregnancy influencing motor and autonomic functions. Change in the pattern of fetal heart rate and movements monitored electronically have been related to fetal neuro behavioural development (DiPietro 1996) and atypical neurodevelopment has been shown in fetuses that exhibit other indicators of neurologic compromise (Hepper 1995). In a publication from Egypt, Kirskey 1991 also reported a positive association between maternal zinc status during the second trimester of pregnancy and newborn behaviour.

It is plausible that the effect of zinc supplementation would vary among different population groups depending on their nutritional status, with any effect likely to be more apparent in women from the developing world. Currently, UNICEF is already promoting antenatal use of multiple‐micronutrient supplementation, including zinc, to all pregnant women in developing countries (Nepal 2003).

The aim of this review is to systematically review all randomised controlled trials of zinc supplementation in pregnancy and to evaluate the role of zinc as it relates to pregnancy, labour and birth as well as to maternal and infant health and well being.

Objectives

To compare the effects on maternal, fetal, neonatal and infant outcomes in healthy pregnant women receiving zinc supplementation, no zinc supplementation, or placebo.
To assess the above outcomes in a subgroup analysis reviewing studies performed in women who are, or are likely to be, zinc deficient.

Methods

Criteria for considering studies for this review

Types of studies

Randomised trials of zinc supplementation versus no zinc supplementation or placebo administration during pregnancy, earlier than 27 weeks' gestation. Quasi‐randomised controlled trials have been excluded. We intended to include studies presented only as abstracts, if they provided enough information or, if necessary, by contacting authors to analyse them against criteria; we did not find such studies.

Types of participants

Normal pregnant women with no systemic illness. Women may have had normal zinc levels or they may have been, or likely to have been, zinc deficient.

Types of interventions

Routine zinc supplementation versus no zinc supplementation, or placebo.

Types of outcome measures

We have included outcomes related to clinical complications of pregnancy on maternal, fetal, neonatal and infant outcomes. We have not included data related to biochemical outcomes or studies reporting only biochemical outcomes.

Primary outcomes

Maternal and pregnancy outcomes

Preterm labour or birth (less than 37 weeks), or both

Neonatal outcomes

Stillbirth or neonatal death
Birthweight
Small‐for‐gestational age (birthweight less than 10th centile for gestational age)
Low birthweight (less than 2.5 kg)

Secondary outcomes

Maternal and pregnancy outcomes

Antepartum haemorrhage
Pregnancy‐induced hypertension
Prelabour rupture of membranes
Post‐term pregnancy
Induction of labour
Any maternal infection
Meconium in liquor
Caesarean section
Instrumental vaginal birth
Retained placenta
Postpartum haemorrhage
Smell dysfunction
Taste dysfunction

Fetal neurodevelopmental assessment

Baseline fetal heart rate
Baseline variability
Number of accelerations
Number of fetal movements
Fetal activity level (minutes)
Movement amplitude

Neonatal outcomes

Gestational age at birth
High birthweight (more than 4.5 kg)
Apgar score of less than five at five minutes
Head circumference
Hypoxia
Neonatal sepsis
Neonatal jaundice
Respiratory distress syndrome
Neonatal intraventricular haemorrhage
Necrotising enterocolitis
Neonatal length of hospital stay
Congenital malformation (non‐prespecified outcome)

Infant/child outcomes

Episodes of disease
Weight for age Z‐score
Weight for height Z‐score
Mid‐upper arm circumference
Mental development index
Psychomotor development index
Other measures of infant or child development

Search methods for identification of studies

The following methods section of this review is based on a standard template used by the Cochrane Pregnancy and Childbirth Group.

Electronic searches

We searched the Cochrane Pregnancy and Childbirth Group's Trials Register by contacting the Trials Search Co‐ordinator (31 October 2014).

The Cochrane Pregnancy and Childbirth Group’s Trials Register is maintained by the Trials Search Co‐ordinator and contains trials identified from:

monthly searches of the Cochrane Central Register of Controlled Trials (CENTRAL);
weekly searches of MEDLINE (OVID);
weekly searches of Embase (OVID);
handsearches of 30 journals and the proceedings of major conferences;
weekly current awareness alerts for a further 44 journals plus monthly BioMed Central email alerts.

Details of the search strategies for CENTRAL, MEDLINE, Embase and CINAHL, the list of handsearched journals and conference proceedings, and the list of journals reviewed via the current awareness service can be found in the ‘Specialized Register’ section within the editorial information about the Cochrane Pregnancy and Childbirth Group.

Trials identified through the searching activities described above are each assigned to a review topic (or topics). The Trials Search Co‐ordinator searches the register for each review using the topic list rather than keywords.

Searching other resources

We searched the references lists of retrieved studies.

We did not apply any language or date restrictions.

Data collection and analysis

For methods used in the previous version of this review, see Mori 2012.

For this update, the following methods, which are based on a standard template used by the Cochrane Pregnancy and Childbirth Group, were used to assess the eight new reports that were identified as a result of the updated search.

Selection of studies

Two review authors Erika Ota (EO), and Celine Miyazaki (CM) independently assessed for inclusion all the potential studies we identified as a result of the search strategy. We resolved any disagreements through discussion.

Data extraction and management

We designed a form to extract data. For eligible studies, EO and CM extracted the data using the agreed form. We planned to resolve any discrepancies through discussion or, if required, we would have consulted Rintaro Mori (RM). We entered data into Review Manager software (RevMan 2014) and checked for accuracy.

When information regarding any of the above was unclear, we attempted to contact authors of the original reports to provide further details.

Assessment of risk of bias in included studies

EO and CM independently assessed risk of bias for each study using the criteria outlined in the Cochrane Handbook for Systematic Reviews of Interventions (Higgins 2011). We resolved any disagreement by discussion. PM and RM independently re‐assessed risk of bias using the updated format newly required for all the studies already included in the previous version due to changes in methods (Higgins 2011).

(1) Random sequence generation (checking for possible selection bias)

We described for each included study the method used to generate the allocation sequence in sufficient detail to allow an assessment of whether it should produce comparable groups.

We assessed the method as:

low risk of bias (any truly random process, e.g. random number table; computer random number generator);
high risk of bias (any non‐random process, e.g. odd or even date of birth; hospital or clinic record number);
unclear risk of bias.

(2) Allocation concealment (checking for possible selection bias)

We described for each included study the method used to conceal allocation to interventions prior to assignment and assessed whether intervention allocation could have been foreseen in advance of, or during recruitment, or changed after assignment.

We assessed the methods as:

low risk of bias (e.g. telephone or central randomisation; consecutively numbered sealed opaque envelopes);
high risk of bias (open random allocation; unsealed or non‐opaque envelopes, alternation; date of birth);
unclear risk of bias.

(3.1) Blinding of participants and personnel (checking for possible performance bias)

We described for each included study the methods used to blind study participants and personnel from knowledge of which intervention a participant received. We considered that studies were at low risk of bias if they were blinded, or if we judged that the lack of blinding would be unlikely to affect results. We assessed blinding separately for different outcomes or classes of outcomes.

We assessed the methods as:

low, high or unclear risk of bias for participants;
low, high or unclear risk of bias for personnel.

(3.2) Blinding of outcome assessment (checking for possible detection bias)

We described for each included study the methods used, if any, to blind outcome assessors from knowledge of which intervention a participant received. We assessed blinding separately for different outcomes or classes of outcomes.

We assessed methods used to blind outcome assessment as:

low, high or unclear risk of bias.

(4) Incomplete outcome data (checking for possible attrition bias due to the amount, nature and handling of incomplete outcome data)

We described for each included study, and for each outcome or class of outcomes, the completeness of data including attrition and exclusions from the analysis. We stated whether attrition and exclusions were reported and the numbers included in the analysis at each stage (compared with the total randomised participants), reasons for attrition or exclusion where reported, and whether missing data were balanced across groups or were related to outcomes. Where sufficient information was reported, or supplied by the trial authors, we re‐included missing data in the analyses which we undertook.

We assessed methods as:

low risk of bias (e.g. no missing outcome data; missing outcome data balanced across groups);
high risk of bias (e.g. numbers or reasons for missing data imbalanced across groups; ‘as treated’ analysis done with substantial departure of intervention received from that assigned at randomisation);
unclear risk of bias.

(5) Selective reporting (checking for reporting bias)

We described for each included study how we investigated the possibility of selective outcome reporting bias and what we found.

We assessed the methods as:

low risk of bias (where it is clear that all of the study’s pre‐specified outcomes and all expected outcomes of interest to the review have been reported);
high risk of bias (where not all the study’s pre‐specified outcomes have been reported; one or more reported primary outcomes were not pre‐specified; outcomes of interest are reported incompletely and so cannot be used; study fails to include results of a key outcome that would have been expected to have been reported);
unclear risk of bias.

(6) Other bias (checking for bias due to problems not covered by (1) to (5) above)

We described for each included study any important concerns we have about other possible sources of bias.

We assessed whether each study was free of other problems that could put it at risk of bias:

low risk of other bias;
high risk of other bias;
unclear whether there is risk of other bias.

(7) Overall risk of bias

We made explicit judgements about whether studies were at high risk of bias, according to the criteria given in the Handbook (Higgins 2011). With reference to (1) to (6) above, we assessed the likely magnitude and direction of the bias and whether we considered it is likely to impact on the findings. We explored the impact of the level of bias through undertaking sensitivity analyses ‐ seeSensitivity analysis.

For this update the quality of the evidence was assessed using the GRADE approach (Schunemann 2009) in order to assess the quality of the body of evidence relating to the following primary outcomes for the main comparisons.

Preterm labour or birth (less than 37 weeks), or both.
Stillbirth or neonatal death.
Birthweight.
Small‐for‐gestational age (birthweight less than 10th centile for gestational age).
Low birthweight (less than 2.5 kg).

The GRADEprofiler (GRADE 2014) was used to import data from Review Manager 5.3 (RevMan 2014) in order to create 'Summary of findings' tables. A summary of the intervention effect and a measure of quality for each of the above outcomes was produced using the GRADE approach. The GRADE approach uses five considerations (study limitations, consistency of effect, imprecision, indirectness and publication bias) to assess the quality of the body of evidence for each outcome. The evidence can be downgraded from 'high quality' by one level for serious (or by two levels for very serious) limitations, depending on assessments for risk of bias, indirectness of evidence, serious inconsistency, imprecision of effect estimates or potential publication bias.

Measures of treatment effect

Dichotomous data

For dichotomous data, we presented results as summary risk ratio with 95% confidence intervals.

Continuous data

For continuous data, we used the mean difference if outcomes were measured in the same way between trials. If necessary, we planned to use the standardised mean difference to combine trials that measured the same outcome, but used different methods.

Unit of analysis issues

Cluster‐randomised trials

We planned to include cluster‐randomised trials in the analyses along with individually‐randomised trials. We would have adjusted their sample sizes or standard errors using the methods described in the Handbook using an estimate of the intracluster correlation co‐efficient (ICC) derived from the trial (if possible), from a similar trial or from a study of a similar population. Had we used ICCs from other sources, we would have reported this and conducted sensitivity analyses to investigate the effect of variation in the ICC. We included one cluster‐randomised trial (Nepal 2003) ‐ analyses adjusted for clustering were presented in study reports and so we did not need to perform the above additional calculations for these study results

We synthesised the relevant information from Nepal 2003 and the individually‐randomised trials. We considered it reasonable to combine the results from both as there was little heterogeneity between the study designs and the interaction between the effect of intervention and the choice of randomisation unit was considered to be unlikely.

If necessary, we would have acknowledged heterogeneity in the randomisation unit and performed a subgroup analysis to investigate the effects of the randomisation unit.

Cross‐over trials were not considered eligible for this review.

Dealing with missing data

For included studies, we noted levels of attrition. We explored the impact of including studies with high levels of missing data in the overall assessment of treatment effect by using sensitivity analysis.

For all outcomes, we carried out analyses, as far as possible, on an intention‐to‐treat basis, i.e. we attempted to include all participants randomised to each group in the analyses, and all participants were analysed in the group to which they had been allocated, regardless of whether or not they received the allocated intervention. The denominator for each outcome in each trial was the number randomised minus any participants whose outcomes were known to be missing.

Assessment of heterogeneity

We assessed statistical heterogeneity in each meta‐analysis using the Tau², I² and Chi² statistics. We regarded heterogeneity as substantial if an I² was greater than 30% and either the Tau² was greater than zero, or there was a low P value (less than 0.10) in the Chi² test for heterogeneity.

Assessment of reporting biases

When there were 10 or more studies in a meta‐analysis, we investigated reporting biases (such as publication bias) using funnel plots. We assessed funnel plot asymmetry visually, and used formal tests for funnel plot asymmetry. We performed exploratory analyses to investigate any asymmetry we detected.

Data synthesis

We carried out statistical analysis using the Review Manager software (RevMan 2014). We used fixed‐effect meta‐analysis for combining data where it was reasonable to assume that studies were estimating the same underlying treatment effect: i.e. where trials were examining the same intervention, and the trials’ populations and methods were judged sufficiently similar. If there was clinical heterogeneity sufficient to expect that the underlying treatment effects differed between trials, or if substantial statistical heterogeneity was detected, we used random‐effects meta‐analysis to produce an overall summary when an average treatment effect across trials was considered clinically meaningful. The random‐effects summary was treated as the average range of possible treatment effects and we discussed the clinical implications of treatment effects differing between trials. If the average treatment effect was not clinically meaningful we did not combine trials.

Where we used random‐effects analyses, the results were presented as the average treatment effect with 95% confidence intervals, and the estimates of Tau² and I².

Subgroup analysis and investigation of heterogeneity

When we identified substantial heterogeneity, we investigated it using subgroup analyses and sensitivity analyses. We considered whether an overall summary was meaningful, and when it was, used random‐effects analysis to produce it.

We carried out the following subgroup analysis by incorporating zinc status as subgroups as part of the primary comparison.

Risk of populations (population with no or low risk of zinc deficiency versus population with assumed risk of zinc deficiency).
Study settings (studies conducted in high‐income settings versus low‐income settings).

The primary outcomes were used in the subgroup analysis.

We assessed differences between subgroups by interaction tests. For random‐effects and fixed‐effect meta‐analyses using methods other than inverse variance, we assessed differences between subgroups by interaction tests.

Sensitivity analysis

We carried out sensitivity analysis to explore the effects of adequate allocation concealment, but found that restricting to only trials with adequate allocation concealment made very little difference to the results for the primary outcomes.

Results

Description of studies

Results of the search

In this update, we found an additional seven reports. We added two reports for one new randomised controlled trial (Egypt 2014) to make a total of 21 included trials. We excluded one new trial (Naher 2012) and added two new reports each for the included studies Indonesia 1999 (Prawirohartono 2011; Prawirohartono 2013), and Ghana 2009 (Saaka 2009; Saaka 2012).

Included studies

We included 21 RCTs involving over 17,000 women and their babies. See table of Characteristics of included studies for details.

Participants and settings

Eighteen studies included women from low‐ and middle‐income settings. One of the four studies in the higher‐income or mixed‐income settings only recruited women at risk of giving birth to small‐for‐gestational age babies (UK 1991a).

Baseline zinc concentrations and nutritional status

Women in most of the studies had, or were likely to have low zinc concentrations and low nutritional status. It is difficult to assess zinc status and most studies have assumed that pregnant women from low‐income groups would be low in zinc as part of their overall poor nutritional status. Where studied, the improvement in serum zinc concentrations in the supplemented group supports this assumption (Bangladesh 2000; Peru 1999). The only studies likely to have included women with normal zinc concentrations were UK 1989; UK 1991a; UK 1991b.

Dosage of zinc supplementation

The dose of daily zinc supplementation ranged from 5 mg (China 2001) to 44 mg zinc per day (Denmark 1996). Some women in S Africa 1985 had doses of up to 90 mg zinc per day.

Duration of supplementation

Women were supplemented from before conception in Nepal 2003 with the shortest duration being from 26 completed weeks' gestation in some women in USA 1983; and USA 1985.

Types of interventions

Most trials (15/21) compared zinc with placebo (Bangladesh 2000; China 2001; Chile 2001; Denmark 1996; Egypt 2014; Ghana 2009; Iran 2010; Pakistan 2005; S Africa 1985; UK 1989; UK 1991a; USA 1983; USA 1985; USA 1989; USA 1995). Two trials (Peru 1999; Peru 2004) compared zinc with non‐zinc supplement (iron plus folate). In some trials (seeCharacteristics of included studies table), all women were also given iron, folate or vitamins or combinations of these. Four trials (Egypt 2014; Indonesia 1999; Indonesia 2001; Nepal 2003) had more than two arms, so these trials were analysed to compare women who received zinc with women who did not.

Nepal 2003 was a cluster‐RCT ‐ analyses adjusted for clustering were presented in study reports and so we did not need to perform additional calculations for these study results.

Adherence to treatment

Two studies (Chile 2001; Denmark 1996) excluded women who did not comply with their treatment (85% and 60% compliance respectively) and the other 19 studies included or probably included women in the analysis who did not comply. Of the latter group, two studies (UK 1991a; USA 1983) presented at least some results separately for those women who complied and those who did not comply. Adherence was generally reported to be over 70%, except for Pakistan 2005; UK 1989; UK 1991a, where it was 50% to nearly 70%.

Excluded studies

We excluded 16 studies. See table of Characteristics of excluded studies for details.

Risk of bias in included studies

Risk of bias for included studies is summarised in Figure 1 and Figure 2.

Zinc supplementation versus no zinc (with or without placebo) for improving pregnancy and infant outcome
Population: Normal pregnant women with no systemic illness Settings: Bangladesh, Chile, China, Denmark, Egypt, Ghana, Indonesia, Iran, Nepal, Pakistan, Peru, South Africa, UK, USA Intervention: Zinc supplementation versus no zinc (with or without placebo)
Outcomes	*Illustrative comparative risks (95% CI)**		Relative effect (95% CI)	No of Participants (studies)	Quality of the evidence (GRADE)	Comments
	Assumed risk	Corresponding risk
	Control	Zinc supplementation versus no zinc (with or without placebo)
Preterm birth	Study population		RR 0.86 (0.76 to 0.97)	7637 (16 studies)	⊕⊕⊕⊝ moderate¹
	129 per 1000	111 per 1000 (98 to 125)
	Moderate
	100 per 1000	86 per 1000 (76 to 97)
Stillbirth or neonatal death	Study population		RR 1.12 (0.86 to 1.46)	5100 (8 studies)	⊕⊕⊝⊝ low^1,2
	40 per 1000	45 per 1000 (34 to 58)
	Moderate
	25 per 1000	28 per 1000 (22 to 37)
Birthweight		The mean birthweight in the intervention groups was 0.9lower (22.2 lower to 24.0 higher)		6757 (17 studies)	⊕⊕⊝⊝ low^1,2
Small‐for‐gestational age	Study population		RR 1.02 (0.94 to 1.11)	4252 (8 studies)	⊕⊕⊕⊝ moderate¹
	265 per 1000	270 per 1000 (249 to 294)
	Moderate
	108 per 1000	110 per 1000 (102 to 120)
Low birthweight	Study population		RR 0.93 (0.78 to 1.12)	5643 (14 studies)	⊕⊕⊕⊝ moderate¹
	196 per 1000	182 per 1000 (153 to 219)
	Moderate
	119 per 1000	111 per 1000 (93 to 133)
The basis for the assumed risk* (e.g. the median control group risk across studies) is provided in footnotes. The corresponding risk (and its 95% confidence interval) is based on the assumed risk in the comparison group and the relative effect of the intervention (and its 95% CI). CI: Confidence interval; RR: Risk ratio;
GRADE Working Group grades of evidence High quality: Further research is very unlikely to change our confidence in the estimate of effect. Moderate quality: Further research is likely to have an important impact on our confidence in the estimate of effect and may change the estimate. Low quality: Further research is very likely to have an important impact on our confidence in the estimate of effect and is likely to change the estimate. Very low quality: We are very uncertain about the estimate.

Date	Event	Description
31 October 2014	New search has been performed	Search updated. Seven new reports identified from the updated search: two reports of one new trial included (Egypt 2014); one new trial excluded (Naher 2012) and four new reports of existing trials added. Methods have been updated. A 'Summary of findings' table incorporated.
31 October 2014	New citation required but conclusions have not changed	The inclusion of one new trial (Egypt 2014) did not change the conclusions.
9 November 2011	New search has been performed	Search updated. Three new trials included (China 2001; Ghana 2009; Iran 2010) and four new trials excluded (Mahmoudian 2005; Van Vliet 2001; Villamor 2006; Yalda 2010).
9 November 2011	New citation required but conclusions have not changed	New authors helped to update this review.
1 July 2011	Amended	Search updated. Thirteen trial reports added to Studies awaiting classification.
6 November 2008	Amended	Converted to new review format.
20 December 2006	New search has been performed	Search updated. Nine new studies have been added to the original seven included studies, plus one previously excluded study (USA 1985) has now been included, making a total of 17 studies included in the 2006 update. A total of 11 studies have been excluded in this update and two studies have been placed in Studies awaiting classification. The Background and Methods sections have been expanded in this update, and additional outcomes have been added. The title has been changed from 'Zinc supplementation in pregnancy' to 'Zinc supplementation for improving pregnancy and infant outcome'. The conclusions regarding the effect of zinc supplementation on reducing preterm birth have been slightly strengthened.

Outcome or subgroup title	No. of studies	No. of participants	Statistical method	Effect size
1 Preterm birth	16	7637	Risk Ratio (M‐H, Fixed, 95% CI)	0.86 [0.76, 0.97]
1.1 Low zinc or nutrition	14	7099	Risk Ratio (M‐H, Fixed, 95% CI)	0.87 [0.77, 0.98]
1.2 Normal zinc or nutrition	2	538	Risk Ratio (M‐H, Fixed, 95% CI)	0.64 [0.31, 1.32]
2 Stillbirth or neonatal death	8	5100	Risk Ratio (M‐H, Fixed, 95% CI)	1.12 [0.86, 1.46]
2.1 Low zinc or nutrition: stillbirth or neonatal death	4	1364	Risk Ratio (M‐H, Fixed, 95% CI)	1.57 [0.83, 2.98]
2.2 Low zinc or nutrition: stillbirth or deaths in first 7 days	1	1555	Risk Ratio (M‐H, Fixed, 95% CI)	1.03 [0.71, 1.51]
2.3 Low zinc or nutrition: deaths from 0 to 28 days	1	1498	Risk Ratio (M‐H, Fixed, 95% CI)	1.08 [0.68, 1.71]
2.4 Normal zinc or nutrition: stillbirth or neonatal death	3	683	Risk Ratio (M‐H, Fixed, 95% CI)	0.93 [0.24, 3.65]
3 Birthweight	17	6757	Mean Difference (IV, Fixed, 95% CI)	0.90 [‐22.23, 24.02]
3.1 Low zinc or nutrition	13	5103	Mean Difference (IV, Fixed, 95% CI)	‐9.87 [‐35.70, 15.96]
3.2 Normal zinc or nutrition	4	1654	Mean Difference (IV, Fixed, 95% CI)	44.46 [‐7.49, 96.41]
4 Small‐for‐gestational age	8	4252	Risk Ratio (M‐H, Fixed, 95% CI)	1.02 [0.94, 1.11]
4.1 Low zinc or nutrition	7	4200	Risk Ratio (M‐H, Fixed, 95% CI)	1.03 [0.95, 1.12]
4.2 Normal zinc or nutrition	1	52	Risk Ratio (M‐H, Fixed, 95% CI)	0.24 [0.05, 1.10]
5 Low birthweight	14	5643	Risk Ratio (M‐H, Random, 95% CI)	0.93 [0.78, 1.12]
5.1 Low zinc or nutrition	11	4964	Risk Ratio (M‐H, Random, 95% CI)	0.93 [0.75, 1.14]
5.2 Normal zinc or nutrition	3	679	Risk Ratio (M‐H, Random, 95% CI)	0.88 [0.58, 1.36]
6 Antepartum haemorrhage	1		Risk Ratio (M‐H, Fixed, 95% CI)	Subtotals only
6.1 Second trimester	1	1206	Risk Ratio (M‐H, Fixed, 95% CI)	1.59 [0.57, 4.45]
6.2 Third trimester	1	1206	Risk Ratio (M‐H, Fixed, 95% CI)	0.96 [0.39, 2.33]
7 Pregnancy hypertension or pre‐eclampsia	7	2975	Risk Ratio (M‐H, Fixed, 95% CI)	0.83 [0.64, 1.08]
8 Prelabour rupture of membranes	2	1691	Risk Ratio (M‐H, Fixed, 95% CI)	0.93 [0.78, 1.11]
9 Post‐term birth	3	1554	Risk Ratio (M‐H, Fixed, 95% CI)	1.09 [0.74, 1.60]
10 Induction of labour	1	52	Risk Ratio (M‐H, Fixed, 95% CI)	0.27 [0.10, 0.73]
11 Any maternal infection	3	1185	Risk Ratio (M‐H, Fixed, 95% CI)	1.06 [0.74, 1.53]
12 Meconium in liquor	2	1385	Risk Ratio (M‐H, Fixed, 95% CI)	1.16 [0.86, 1.56]
13 Caesarean section	6	2164	Risk Ratio (M‐H, Random, 95% CI)	0.95 [0.58, 1.53]
14 Instrumental vaginal birth	1	1206	Risk Ratio (M‐H, Fixed, 95% CI)	1.12 [0.79, 1.59]
15 Retention of placenta	1	179	Risk Ratio (M‐H, Fixed, 95% CI)	6.62 [0.83, 52.71]
16 Postpartum haemorrhage	3	718	Risk Ratio (M‐H, Fixed, 95% CI)	1.33 [0.78, 2.26]
17 Smell dysfunction	1	170	Risk Ratio (M‐H, Fixed, 95% CI)	1.01 [0.55, 1.86]
18 Taste dysfunction	1	170	Risk Ratio (M‐H, Fixed, 95% CI)	0.74 [0.36, 1.50]
19 Fetal heart rate (beats/minute)	1	176	Mean Difference (IV, Fixed, 95% CI)	‐1.20 [‐3.31, 0.91]
20 Fetal heart rate variability (beats/minute)	1	176	Mean Difference (IV, Fixed, 95% CI)	0.60 [0.04, 1.16]
21 Number of fetal accelerations	1	176	Mean Difference (IV, Fixed, 95% CI)	1.9 [0.91, 2.89]
22 Number of fetal movement bouts	1	176	Mean Difference (IV, Fixed, 95% CI)	1.70 [‐2.53, 5.93]
23 Fetal activity level	1	176	Mean Difference (IV, Fixed, 95% CI)	‐0.30 [‐2.66, 2.06]
24 Fetal movement amplitude	1	176	Mean Difference (IV, Fixed, 95% CI)	0.20 [‐0.79, 1.19]
25 Gestational age at birth	7	2857	Mean Difference (IV, Fixed, 95% CI)	0.08 [‐0.07, 0.22]
26 High birthweight	5	2837	Risk Ratio (M‐H, Fixed, 95% CI)	1.00 [0.84, 1.18]
27 Five‐minute Apgar score less than 5	2	1692	Risk Ratio (M‐H, Fixed, 95% CI)	1.02 [0.26, 4.03]
28 Infant head circumference (cm)	7	3991	Mean Difference (IV, Random, 95% CI)	‐0.03 [‐0.17, 0.11]
29 Blue or floppy (neonatal hypoxia)	1	179	Risk Ratio (M‐H, Fixed, 95% CI)	5.67 [0.70, 46.18]
30 Neonatal sepsis	2	736	Risk Ratio (M‐H, Fixed, 95% CI)	0.17 [0.03, 1.01]
31 Neonatal jaundice	1	179	Risk Ratio (M‐H, Fixed, 95% CI)	0.95 [0.20, 4.56]
32 Respiratory distress syndrome	2	1136	Risk Ratio (M‐H, Fixed, 95% CI)	0.68 [0.40, 1.14]
33 Neonatal intraventricular haemorrhage	1	580	Risk Ratio (M‐H, Fixed, 95% CI)	0.97 [0.14, 6.86]
34 Necrotising enterocolitis	1	580	Risk Ratio (M‐H, Fixed, 95% CI)	1.95 [0.18, 21.34]
35 Neonatal hospital stay	1	580	Mean Difference (IV, Fixed, 95% CI)	‐1.1 [‐2.39, 0.19]
36 Congenital malformation	6	1240	Risk Ratio (M‐H, Fixed, 95% CI)	0.67 [0.33, 1.34]
37 Diarrhoea (episodes/infant over 6 months)	1		Mean Difference (IV, Fixed, 95% CI)	Subtotals only
37.1 Acute diarrhoea	1	410	Mean Difference (IV, Fixed, 95% CI)	‐0.40 [‐0.79, ‐0.01]
37.2 Persistent diarrhoea	1	410	Mean Difference (IV, Fixed, 95% CI)	0.0 [‐0.13, 0.13]
38 Dysentery (episodes/infant over 6 months)	1	410	Mean Difference (IV, Fixed, 95% CI)	‐0.06 [‐0.12, 0.00]
39 Cough (episodes/infant over 6 months)	1	410	Mean Difference (IV, Fixed, 95% CI)	‐0.20 [‐0.56, 0.16]
40 Acute lower respiratory infection (episodes/infant over 6 months)	1	410	Mean Difference (IV, Fixed, 95% CI)	‐0.10 [‐0.34, 0.14]
41 Impetigo (episodes/infant over 6 months)	1	410	Mean Difference (IV, Fixed, 95% CI)	‐0.30 [‐0.44, ‐0.16]
42 Infant weight‐for‐age (Z‐score)	2		Mean Difference (IV, Random, 95% CI)	Subtotals only
42.1 Z‐score at 6 months	2	304	Mean Difference (IV, Random, 95% CI)	0.20 [‐0.19, 0.59]
42.2 Z‐score at 13 months	1	168	Mean Difference (IV, Random, 95% CI)	‐0.40 [‐0.70, ‐0.10]
43 Infant weight‐for‐height (Z‐score)	1	136	Mean Difference (IV, Fixed, 95% CI)	‐0.05 [‐0.33, 0.23]
44 Infant mid‐upper arm circumference (mm)	3	1844	Mean Difference (IV, Fixed, 95% CI)	0.74 [‐0.17, 1.65]
45 Infant mental development index	1	168	Mean Difference (IV, Fixed, 95% CI)	‐3.30 [‐6.51, ‐0.09]
46 Infant psychomotor development index	1	168	Mean Difference (IV, Fixed, 95% CI)	‐7.0 [‐11.92, ‐2.08]
47 Infant approach	1	168	Mean Difference (IV, Fixed, 95% CI)	0.10 [‐0.38, 0.58]
48 Infant emotional tone	1	168	Mean Difference (IV, Fixed, 95% CI)	‐0.65 [‐1.13, ‐0.17]
49 Infant activity	1	168	Mean Difference (IV, Fixed, 95% CI)	‐0.10 [‐0.43, 0.23]
50 Infant co‐operation	1	168	Mean Difference (IV, Fixed, 95% CI)	‐0.60 [‐1.16, ‐0.04]
51 Infant vocalisation	1	168	Mean Difference (IV, Fixed, 95% CI)	‐0.08 [‐0.54, 0.38]
52 Differential abilities score at 5 years	1		Mean Difference (IV, Fixed, 95% CI)	Subtotals only
52.1 Non‐verbal ability	1	355	Mean Difference (IV, Fixed, 95% CI)	‐2.40 [‐5.70, 0.90]
52.2 Verbal ability	1	355	Mean Difference (IV, Fixed, 95% CI)	‐0.30 [‐2.56, 1.96]
52.3 General conceptual ability, IQ	1	355	Mean Difference (IV, Fixed, 95% CI)	‐1.10 [‐3.74, 1.54]
53 Visual sequential memory score	1	355	Mean Difference (IV, Fixed, 95% CI)	‐0.80 [‐2.24, 0.64]
54 Auditory sequential memory score	1	355	Mean Difference (IV, Fixed, 95% CI)	0.60 [‐0.65, 1.85]
55 Knox cube score	1	355	Mean Difference (IV, Fixed, 95% CI)	0.10 [‐0.19, 0.39]
56 Gross motor scale score	1	355	Mean Difference (IV, Fixed, 95% CI)	‐2.0 [‐4.79, 0.79]
57 Grooved pegboard score	1		Mean Difference (IV, Fixed, 95% CI)	Subtotals only
57.1 Dominant hand	1	355	Mean Difference (IV, Fixed, 95% CI)	2.5 [‐1.26, 6.26]
57.2 Non‐dominant hand	1	355	Mean Difference (IV, Fixed, 95% CI)	1.20 [‐2.71, 5.11]
58 Intelligence quotient of infants at 54 months	1	181	Mean Difference (IV, Fixed, 95% CI)	‐0.40 [‐3.33, 2.53]

Methods	A double‐blind, randomised, placebo‐controlled trial.
Participants	559 pregnant women between 12 and 16 weeks' gestation, from Dhaka city slums. The 446 women who completed follow‐up had a mean baseline serum zinc level of 15.3 [SD 4.3] µmol/L (similar to those lost to follow‐up). Energy intakes were low at 4 months' gestation (median 6065 kJ/day). A total of 559 pregnant women from selected areas of Dhaka city slums were identified between 12 and 16 weeks of gestation through an established pregnancy identification system between March and June 1996. Women were included if they remain at or near their residences in Dhaka for the delivery without established medical risk for reduced or excessive birthweight (e.g. hypertension, renal disease, or diabetes). The 446 women who completed follow‐up had a mean baseline serum zinc level of 15.3 [SD 4.3] µmol/L (similar to those lost to follow‐up). Energy intakes were low at 4 months' gestation (median 6065 kJ/day).
Interventions	Zinc was given twice the recommended daily intake during the last 2 trimesters of pregnancy. The zinc tablets contained (31.0 mg Zn/tablet; range: 28.6–32.6) and placebo tablets contained (0.0 mg Zn/tablet; range: 0.0–0.1) were was verified and confirmed by 2 independent laboratories. The placebo was a cellulose tablet indistinguishable from the zinc supplement in both appearance and taste. Health workers provided a 1‐week supply of zinc or placebo tablets (ACME Ltd, Dhaka) to the houses of the women weekly and instructed the women to consume 1 tablet daily between meals and not together with other vitamin or mineral supplements. Compliance was assessed by counting the remaining tablets in each strip at the next visit. Unannounced compliance checks between regular visits were performed monthly in subsamples of 10% of the study participant. Zinc: 30 mg elemental zinc/day (n = 269 [214]) . Placebo: n = 290 [232]).
Outcomes	Maternal outcomes Serum zinc concentrations at 7 months' gestation; haemoglobin concentrations at 7 months' gestation; blood pressure at 7 months' gestation; preterm birth and gestational age; stillbirth. Neonatal outcomes Birthweight; low birthweight, < 2500 g, < 2000 g, < 1500 g; gestational age (weeks); prematurity,< 37 weeks, < 32 weeks; small‐for‐gestational age; length (cm), head circumference (cm), chest circumference (cm), and mid‐upper arm circumference (mm).
Notes	Adherence: percentage of days during follow‐up that a woman reported having consumed a supplement was 86%. Final sample size of 410 infants was sufficient to detect a 110 g difference in birthweight.
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	"computer‐generated random letter assignment."
Allocation concealment (selection bias)	Unclear risk	"randomly assigned" ‐ no details given regarding allocation concealment.
Blinding of participants and personnel (performance bias) All outcomes	Low risk	Both investigators and participants were blinded.
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Not specifically mentioned but assessors were also likely to have been blinded.
Incomplete outcome data (attrition bias) All outcomes	Unclear risk	113/559 (20.2%) women were lost to follow‐up before birth; (55 (20.4%) in the zinc group and 58 (20.0%) in the placebo group) ‐ most (60) due to migration out of the area. By 13 months follow‐up, 383 (68.5%) infants remained in the trial, with only 168 of these infants being included in the 13‐month analysis.
Selective reporting (reporting bias)	Unclear risk	Some primary outcomes such as mode of birth not reported.
Other bias	Low risk	No apparent source of other bias.

Methods	A double‐blinded, randomised placebo‐controlled trial.
Participants	Normal healthy middle‐class women were (at least 18 years old) less than 20 weeks pregnant confirmed by scan for their first visit and booked for delivery at Kolding hospital and Horsens hospital, Denmark. Any known intolerance towards zinc, diabetes mellitus, thyrotoxicosis or earlier rhesus immunization were excluded from the trials. The women thought likely to be zinc deficient by the previous study project 'Pregnancy, environment and way of life' in Denmark.
Interventions	Women received 2 tablets of Zinclet@ (44 mg elemental zinc in total) or 2 placebo tablets containing inert substances. They were indistinguishable in appearance and taste. The tablets were prepared by the Gunnar Kjems Aps company. Women were advised to take 2 tablets daily after breakfast and to avoid taking tablets possibly containing iron together with those of the study, as iron reduces zinc uptake. Women were excluded later, if there were any side‐effects caused by the tablets, if they wanted to stop or if she had not taken the tablets for 14 days in all. Zinc: 2 tablets with 44 mg elemental zinc (n = 1000). No zinc: 2 placebo tablets indistinguishable from active tablets (n = 1000).
Outcomes	Maternal outcomes Prelabour rupture of membranes; preterm labour; pre‐eclampsia; antepartum haemorrhage; caesarean section. Neonatal outcomes Low 5‐minute Apgar score; large‐for‐gestational age; small‐for‐gestational age; birthweight (not able to be used in graphs since no SDs provided).
Notes	Adherence: non‐adherers were excluded from the final analysis; reasons included side‐effects from tablets, if a woman wished to stop or if a woman had not taken the tablets for 14 days in all. The authors noted that women did not differ in basic characteristics. There were however, significantly more smokers in the non‐adherers group and thus the numbers in the final analysis related to labour and birth have also excluded smokers.
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Randomisation was performed in successive groups of 10 active and 10 placebos; no further details reported.
Allocation concealment (selection bias)	Unclear risk	Not reported.
Blinding of participants and personnel (performance bias) All outcomes	Low risk	Investigators and mothers were blinded.
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Not reported, but probably done as paper reports that the code was not broken until the end of the study.
Incomplete outcome data (attrition bias) All outcomes	High risk	794/2000 (39.7%); 415 in zinc group and 379 in placebo group.
Selective reporting (reporting bias)	Unclear risk	Not all expected maternal primary outcomes reported, but most primary infant outcomes specified in this review were reported.
Other bias	Unclear risk	Analyses relating to labour and birth excluded smokers.

Methods	A double‐blind, RCT.
Participants	400 pregnant women in Ghana earlier than 16 weeks of gestation who presented themselves for antenatal care and have been screened for their gestational age in the Wa Regional Hospital of the Upper West Region in Ghana. Women who were receiving zinc supplements at any dosage level or were severely anaemic (that is, Hb less than 7.0 g/dL) were excluded. The iron‐zinc and iron‐only supplements were pre‐coded and supplied by Nutricaps pharmaceutical company in the United States of America. The supplements (in the form of capsules) were of the same shape, colour and taste and packaging. The women were advised to take the supplements at least 2 hours before or after meals, and at night, just before going to bed. Compliance was monitored by interviewing all participants after having being enrolled for 4 weeks using structured questionnaire to check the frequency and dosage of supplement intake. A subsample of 213 women at recruitment were assessed for serum ferritin but only 173 were repeated at 34‐36 weeks' gestation. N = 299 for intervention and n = 301 for control allocated. 27 out of 299 of the intervention group and 30 out of 301 of the control group were lost to follow‐up and excluded from the analysis.
Interventions	Women received a combined supplement of 40 mg zinc as zinc gluconate and 40 mg iron as ferrous sulphate and women in the control group received 40 mg elemental iron as ferrous sulphate without zinc content. Both groups received malaria chemoprophylaxis in the form of sulphadoxine pyrimethamine, and 400 μg folic acid. The supplements were taken every other day from enrolment until delivery. Intervention group: 40 mg zinc plus 40 mg iron (n = 299). Control group: 40 mg iron only (n = 301).
Outcomes	Intrauterine growth restriction/small‐for‐gestational age; low birthweight; preterm birth; birthweight; haemoglobin concentration; serum ferritin concentration; plasma zinc concentration.
Notes
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	By computer‐generated random number.
Allocation concealment (selection bias)	Unclear risk	Opaque envelopes.
Blinding of participants and personnel (performance bias) All outcomes	Low risk	The capsules for both intervention and placebo were the same.
Blinding of outcome assessment (detection bias) All outcomes	Unclear risk	No description.
Incomplete outcome data (attrition bias) All outcomes	Unclear risk	27 out of 299 of the intervention group and 30 out of 301 of the control group were lost to follow‐up and excluded from the analysis.
Selective reporting (reporting bias)	Unclear risk	It was not clear if a protocol of this trial had been published prior to the study; no maternal outcomes reported.
Other bias	Low risk	Baseline characteristics were compared, with no significant difference seen between groups.

Methods	A double‐blinded, RCT. A follow‐up study of the infants with a factorial design. A 4‐arm trial.
Participants	230 pregnant women were recruited before 20 weeks of gestational age from 13 adjacent villages in Bogor District, Indonesia. Of 230 women, 179 women remain until delivery and only and only 170 women were enrolled for follow up of infant and mother until 6 months postpartum study. Each woman was supplemented daily during pregnancy until delivery. Exclusion criteria at enrolment were twin pregnancy and congenital abnormalities. Women had mean plasma zinc concentrations of about 11 µmol/L.
Interventions	All women received iron and folic acid (30 mg iron as ferrous fumarate/d and 0.4 mg pteroylglutamic acid/d). In addition, 1 group of women received ß‐carotene (4.5 mg as water‐soluble granulate/d; ß‐carotene group), 1 group received zinc (30 mg zinc as sulphate/day; zinc group), 1 group received ß‐carotene plus zinc (4.5 mg ß‐carotene and 30 mg zinc/d; ß‐carotene + zinc group), and 1 group received only iron and folic acid (control group). Capsules were prepared by the pharmacy of the Gelderse Vallei Hospital (Ede, Netherlands) and given a letter code and the micronutrients were indistinguishable from each other. Compliance, expressed as a proportion of the intended supplements consumed during pregnancy, did not differ among the groups with a mean compliance of > 80% in all groups, and 90% of the women taking > 50% of the intended dose. Iron + folate acid: (n = 41). Iron + folate acid + β‐carotene: (n = 43). Iron + folate acid + zinc: (n = 44). Iron + folate acid + β‐carotene + zinc (n = 42).
Outcomes	Maternal outcomes Preterm birth; caesarean section; prolonged labour; retention of placenta; postpartum haemorrhage; infection; 6‐month serum zinc. Neonatal outcomes Birthweight; low birthweight; congenital malformation; stillbirth/neonatal death; blue/floppy (neonatal hypoxia); jaundice; fever/not drinking; umbilical infection; 6‐month Z‐scores; 6‐month haemoglobin; plasma retinol; plasma zinc.
Notes	Adherence: mean adherence was over 80%.
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Method of sequence generation not reported.
Allocation concealment (selection bias)	Unclear risk	Supplements were prepared by a third party (hospital pharmacy in the Netherlands), but no detail given of how the contents of the bottles were concealed from the investigators or the participants.
Blinding of participants and personnel (performance bias) All outcomes	Low risk	Stated as being "double‐blind"; probably done.
Blinding of outcome assessment (detection bias) All outcomes	Unclear risk	Not reported.
Incomplete outcome data (attrition bias) All outcomes	Unclear risk	Losses to follow‐up: 50/229 (22%) women before giving birth; 136 newborns completed follow‐up at 6 months.
Selective reporting (reporting bias)	Unclear risk	Not all expected maternal primary outcomes reported, but most primary infant outcomes specified in the review were reported.
Other bias	Low risk	No apparent risk of other bias.

Methods	A double‐blind, cluster‐randomised, controlled trial (also factorial design). It was an 1‐5 treatment arms intervention.
Participants	4926 pregnant women and 4130 liveborn infants in a rural plains district of Sarlahi, community in Nepal, which had 426 sectors (communities of about 100‐150 households) ‐ only 2 of the 5 arms (total of 1659 infants) used in this review. This is the same area of Nepal in which we previously recorded evidence of vitamin A, iron, and zinc deficiency among pregnant women. Women who were currently pregnant, breastfeeding a baby less than 9 months old, menopausal, sterilised or widowed were excluded. Supplementation commenced before conception.
Interventions	The sectors were randomly assigned to 1 of 5 treatment arms. The control group was vitamin A (1000 µg retinol equivalents). FA group, vitamin A + folic acid (400 µg). FAFe group, vitamin A + folic acid + iron (60 mg). FAFeZn group, vitamin A + folic acid + iron + zinc (30 mg). MN group, vitamin A + folic acid + iron+ zinc + other micronutrients (10 µg vitamin D, 10 mg vitamin E, 1.6 mg thiamine, 1.8 mg riboflavin, 20 mg niacin, 2.2 mg vitamin B‐6, 2.6 µg vitamin B‐12, 100 mg vitamin C, 65 µg vitamin K, 2.0 mg Cu, 100 mg Mg). The supplements were provided from UNICEF, identical in shape, size, and colour, arrived in Nepal in opaque, sealed, and labelled bottles coded 1–5. The code allocation was kept locked at the Johns Hopkins University, Baltimore. The investigators, field staff, and participants were blinded to the codes throughout the study. Zinc: zinc + iron + folate (n = 858). No zinc: iron + folate (n = 801).
Outcomes	Maternal outcome Preterm birth. Neonatal outcomes Stillbirth or neonatal death; birthweight; chest circumference; head circumference; length; low birthweight; small‐for‐gestational age.
Notes	Adherence: mean adherence was 88%. RRs adjusted for the cluster‐design effects were presented for each of the 5 arms of the RCT.
*Risk of bias*
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Randomised sectors by "drawing numbered identical chips from a hat" (in blocks of 5 within each community).
Allocation concealment (selection bias)	Low risk	Supplements were of identical shape, size and colour and arrived in Nepal in opaque, sealed and labelled bottles coded 1‐5. The code allocation was kept locked at the Johns Hopkins University, Baltimore.
Blinding of participants and personnel (performance bias) All outcomes	Low risk	Participants, investigators, field staff and statisticians were all blinded to the codes throughout the study.
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Investigators, field staff and statisticians were all blinded to the codes throughout the study.
Incomplete outcome data (attrition bias) All outcomes	Unclear risk	155/827 (19%) of infants in the zinc group and 167/872 (19%) in the non‐zinc group were lost to follow‐up or excluded from analysis (infant died, mother refused, home was inaccessible, birthweight was measured more than 72 hours after birth or missing data).
Selective reporting (reporting bias)	Low risk	Most expected outcomes were reported with some exceptions such as mode of birth and postpartum haemorrhage.
Other bias	Low risk	No apparent evidence of other sources of bias apart from a small imbalance between groups in maternal weight (which was adjusted for in the analyses).

Study	Reason for exclusion
An 2001	313 healthy pregnant women at their fifth month of gestation enrolled in the hospital for prenatal care in Beijin, China, were given fortified biscuits containing (vitamin D), (vitamin D + calcium), (vitamin D + calcium + iron), (vitamin D + calcium + iron + zinc). Only 1 woman selected from the same hospital (no fortified biscuits given) as control comparison. The study indicated quasi‐randomised study, which there was no randomised sequence generation performed and allocation was by the order of hospital visits.
Appelbaum 1979	The effect of diet supplementation throughout pregnancy on third‐trimester amniotic fluid growth‐supporting activity was studied in 100 African women; 32 were given zinc supplementation, 22 each animal and vegetable supplements, respectively, and 24 served as control subjects. Zinc level was measured among the groups and the inhibitory of zinc was conducted in vitro testing of the amniotic fluid collected. The study design did not indicate randomised controlled trial and the outcome was the zinc level of amniotic fluid only.
Christian 2001	A placebo‐controlled trial in Nepal was conducted on 202 women who reported to be night blind during pregnancy. They were randomly assigned in a double‐blind manner, stratified on vitamin A, ß‐carotene, or placebo receipt, to receive 25 mg Zn or placebo daily for 3 weeks. The participant selection was on women who had night blindness and there was no prespecified outcomes reported related to pregnancy except for vision restoration of pregnant women.
Fawzi 2005	Pregnant women who were HIV‐infected, who resided in Dar es Salaam, Tanzania, at the time of the baseline interview, and who intended to stay in the city until delivery and for were considered. 400 HIV‐infected pregnant women were between 12 and 27 weeks of gestation were randomly assigned to daily oral supplementation with either 25 mg Zn or placebo between recruitment and 6 weeks after delivery. The population selected for the trial were not in healthy state of condition.
France 2004	Healthy pregnant women (n = 100) receiving prenatal care between 12 and 16 weeks of gestation in the Obstetric Departments of Grenoble and Lyon Hospitals in France participated in a double‐blind, randomised, placebo‐controlled trial. The intervention was micronutrients supplement or placebo. The micronutrients contained vitamin C (60 mg), ß‐carotene (4.8 mg), vitamin E (10 mg), thiamin (1.4 mg), riboflavin (1.6 mg), niacin (15 mg), pantothenic acid (6 mg), folic acid (200 mg), cobalamin (1 mg), Zn (15 mg as citrate), Mg (87.5 mg as glycerophosphate), Ca (100 mg as carbonate). Zinc was not given separately as the main intervention.
Hambidge 1983	A longitudinal study (monthly intervals) in 46 pregnant middle‐income women. 10 of the women received a daily supplement of 15 mg Zn and the rest of the women did not receive any zinc supplement. The design was an observational study with no mention of randomisation or allocation to zinc or no‐zinc groups.
India 1993	90 pregnant women were randomly assigned to control (A) and 120 pregnant women were randomly assigned to zinc treated (B) groups. Group B women were administered a single daily dose of 45 mg zinc as a 200 mg zinc sulphate tablet (Zinfate, Yash Pharma) from the day of reporting till delivery. The control group women were not provided with zinc supplementation. The total number of subjects finally selected in group A served as control was 62, and that in Gp. B, was 106. The study design was a quasi‐randomised study and with large discrepancies in numbers of participants at baseline and follow‐up.
Kynast 1986	A randomly selected study group of 179 pregnant women and a control group of 345 pregnant women were given zinc aspartate. This study investigates the prophylactic effectiveness of zinc replacement in reducing the overall complication rate for both mother and fetus and in particular for large‐for‐date and small‐for‐date infants. The study design was a quasi‐randomised study and allocation was done by alternation.
Mahmoudian 2005	A total of 118 anaemic women were recruited in this randomised controlled trial. Both groups received 100 mg elemental iron daily. The intervention group received an additional dose of 15 mg zinc every day for a period of 12 weeks while the control group received placebo. The participants were all anaemic women and outcomes were haemoglobin concentration only.
Makola 2003	This study was a randomised, placebo‐controlled double‐blind effectiveness trial of a micronutrient‐fortified dietary supplement conducted in pregnant women in Tanzania. Pregnant women who believed that they were between 12 and 34 weeks pregnant were invited to participate in the study. The intervention was micronutrient supplement (orange‐flavoured micronutrient‐fortified powdered beverage mix containing 11 micronutrients, including zinc) in comparison to placebo. Women with gestation greater than 26 weeks were included in the study and zinc was not provided separately as an supplemented intervention.
Naher 2012	A total of 200 pregnant women, age ranging between 18‐40 years and gestational age ranging from 37‐42 weeks were selected for a cross‐sectional study in Bangladesh. Among them, 100 were advised to take 61.8 mg zinc daily and the others did not. The study design was an observational study and not a randomised controlled trial.
Nishiyama 1999	38 Japanese women at the second trimester of pregnancy had haemoglobin concentrations below 11.0 g/dL and 32 of 38 had normocytic erythrocytes. These women were divided into 3 groups, and they were compared for their haematological status and serum IGF‐I levels before and after iron (Group A) or Zn (Group B) or iron plus Zn (Group C) supplementation. The the women were anaemic and the study design was a clinical controlled trial where women could chose 1 of 3 intervention groups.
Nogueira 2003	74 low‐income pregnant adolescents in Brazil ranging from 13‐18 years of age received supplementation of (folic acid + iron), (folic acid + zinc sulphate + iron) or only (iron). The pregnant adolescents were divided into 5 groups. The study method was based on longitudinal design and zinc plasma concentration was measured as to support the folic acid metabolism.
Van Vliet 2001	The study was an open, randomised study, using a cross‐over approach for 2 sources of vitamin A (i.e. liver paste and retinyl palmitate containing oil) and a parallel approach for 3 dose levels of vitamin A (i.e. 3.0, 7.5, and 15 mg vitamin A) for women between 19‐47 years of age. Pregnant women were excluded in the study. The intervention did not use zinc supplements or any zinc constituent supplements.
Villamor 2006	Pregnant women with HIV and the HIV status of their babies was assessed at birth and at 6 weeks postpartum at Dar es Salaam Tanzania. Women 12–27 weeks of gestation were randomly assigned to receive a daily oral dose of 25 mg zinc or placebo from the day of the first prenatal visit until 6 weeks postdelivery. All the participants were infected with HIV.
Yalda 2010	Single‐blind randomised clinical control trial conducted in Kurdistan region, Iraq. 100 anaemic pregnant women were selected to receive, first group (A), supplemented daily with 120 mg iron and second group (B) received 120 mg iron + 22.5 mg zinc. The pregnant women were all diagnosed with anaemia and the outcome were to assess the improvement of anaemic condition.

Trial name or title	Assessment of the effect of zinc supplementation on adverse outcomes of pregnancy.
Methods	Randomised controlled trial.
Participants	Inclusion criteria: gestational age of 12‐16 weeks based on reliable LMP or first trimester ultrasound, lack of history of high‐risk pregnancy, lack of chronic underlying diseases (such as heart disease, HTN, DM). Exclusion criteria: lack of complete treatment or lack of follow‐up.
Interventions	Zinc 30 mg from 12th week of gestation every other day in the intervention group and no zinc is supplemented in the control group.
Outcomes	Gestation, birthweight and other pregnancy and neonatal clinical outcomes.
Starting date	March 2009.
Contact information	Dr Ziba Zahiri (drzibazahiri@gums.ac.ir).
Notes

Zinc supplementation for improving pregnancy and infant outcome

Abstract

Background

Objectives

Search methods

Selection criteria

Data collection and analysis

Main results

Authors' conclusions

Plain language summary

Summary of findings

Background

Description of the condition

Description of the intervention

How the intervention might work

Why it is important to do this review

Objectives

Methods

Criteria for considering studies for this review

Types of studies

Types of participants

Types of interventions

Types of outcome measures

Primary outcomes

Maternal and pregnancy outcomes

Neonatal outcomes

Secondary outcomes

Maternal and pregnancy outcomes

Fetal neurodevelopmental assessment

Neonatal outcomes

Infant/child outcomes

Search methods for identification of studies

Electronic searches

Searching other resources

Data collection and analysis

Selection of studies

Data extraction and management

Assessment of risk of bias in included studies

(1) Random sequence generation (checking for possible selection bias)

(2) Allocation concealment (checking for possible selection bias)

(3.1) Blinding of participants and personnel (checking for possible performance bias)

(3.2) Blinding of outcome assessment (checking for possible detection bias)

(4) Incomplete outcome data (checking for possible attrition bias due to the amount, nature and handling of incomplete outcome data)

(5) Selective reporting (checking for reporting bias)

(6) Other bias (checking for bias due to problems not covered by (1) to (5) above)

(7) Overall risk of bias

Measures of treatment effect

Dichotomous data

Continuous data

Unit of analysis issues

Cluster‐randomised trials

Dealing with missing data

Assessment of heterogeneity

Assessment of reporting biases

Data synthesis

Subgroup analysis and investigation of heterogeneity

Sensitivity analysis

Results

Description of studies

Results of the search

Included studies

Participants and settings

Baseline zinc concentrations and nutritional status

Dosage of zinc supplementation

Duration of supplementation

Types of interventions

Adherence to treatment

Excluded studies

Risk of bias in included studies

Allocation

Blinding

Incomplete outcome data

Selective reporting

Other potential sources of bias

Effects of interventions

Summary of findings for the main comparison

Primary outcomes:

Secondary outcomes

Maternal outcomes

Birthweight and associated outcomes