Birth outcomes and background exposures to select elements, the Longitudinal Investigation of Fertility and the Environment (LIFE)

Abstract

1. INTRODUCTION

Concern continues to mount for adverse impacts resulting from non-occupational or ‘background’ exposures to environmental agents (ACOG and ASRM, 2013) possibly leading to poorer birth outcomes and life-long sequelae (Stillerman et al., 2008). This includes exposure to non-essential elements that confer no physiologic benefit and are known to be toxic at low doses, such as arsenic (As), cadmium (Cd), lead (Pb), and mercury (Hg). While some elements such as chromium (Cr), copper (Cu), selenium (Se), and zinc (Zn) are essential for normal physiologic function, they are also potentially toxic at higher doses (Domingo, 1994; Fairbrother et al., 2007). Exposure is widespread, via diet, contaminated water, and contaminated air, and having been detected in a majority of specimens collected by U.S. biomonitoring studies (CDC, 2014b) including in pregnant women (Woodruff et al., 2011). Given their potential for reproductive toxicity and widespread nature we conducted an exploratory analysis of trace exposures to select elements and birth outcomes.

Pregnant women and their fetuses are at increased risk for adverse effects from environmental agents (Sutton et al., 2010). Several groups reported associations between maternal exposure to non-essential elements and adverse birth outcomes, but most frequently to high doses of As, Cd, Hg, or Pb encountered in the workplace or associated with residence in so-called ‘endemic’ regions (Wigle et al., 2008). Adverse birth outcomes, including preterm delivery (PD) and low birth weight (LBW) increase lifetime risks for myriad morbidities, including neurodevelopmental complications (Mwaniki et al., 2012), poorer intellectual development (Bergvall et al., 2006), and cardiovascular disease and endocrine disorders (Barker, 2004). Even at term, adult mortality is associated with earlier delivery (Crump et al., 2011). Investigators have described associations between background exposures to toxic and essential elements in pregnant women and birth outcomes (Al-Saleh et al., 2014; Gundacker et al., 2010; Kippler et al., 2012a; Kippler et al., 2012b; Kozikowska et al., 2013; Lee et al., 2010; Lin et al., 2011; Menai et al., 2012; Shirai et al., 2010; Tian et al., 2009; van Wijngaarden et al., 2014; Zhang et al., 2004). However, there are few data to assess the impact of preconception exposures; measurements made during pregnancy are subject to within-woman variability concurrent to gestation-related physiologic adaptations (Selevan et al., 2000).

Recent evidence substantiates the transgenerational effects of environmental risk factors (Boekelheide et al., 2012), suggesting an important role for preconception parental exposures on birth outcomes. Either through de novo generation of reactive oxygen species or depletion of anti-oxidants, excess exposure to metals and metalloids can increase oxidative stress (Ercal et al., 2001; Valko et al., 2005), in turn impacting erasure and programming of epigenetic marks - the dynamic pattern of DNA and histone bound methyl- and acetyl- groups that regulates gene expression (Baccarelli and Bollati, 2009; Ho et al., 2012). Epigenetic germ cell alterations are likely heritable, as demonstrated in spermatozoa (Anway et al., 2005) or through maternal lineage (Newbold et al., 1998; Newbold et al., 2000) in vinclozolin or diethylstilbestrol treated mice, respectively. Alternately, modified reprogramming of epigenetic patterns, which begin only hours after fertilization (Reik et al., 2001), may influence fetal growth and development. Furthermore, many metals and metalloids, including Cd, Pb, Hg, and U are estrogenic, whereas others, such as As, are also anti-estrogenic (Dyer, 2007; Iavicoli et al., 2009), which may further influence epigenetic patterning (Crews and McLachlan, 2006), increasing the prospect for preconception impacts on birth outcomes.

To the best of our knowledge no prior studies assessed pre-conception couple-level exposures to these elements. To begin to address the existing data gap, our aim was to identify associations for further confirmation, between birth outcomes and background or ‘trace’ exposures to select metals and metalloids. To accomplish this aim we used couples participating in the Longitudinal Investigation of Fertility and the Environment (LIFE), a prospective cohort investigation of environmental factors and human reproduction.

2. MATERIALS AND METHODS

3. RESULTS

4. DISCUSSION

In this prospective couple-based cohort study we observed modest evidence for associations between exposure to select elements and birth outcomes. We detected no consistent pattern of diminished GA or birth size in association with preconception parental elements, yet paternal exposure tended to be more frequently related to decrements than maternal exposure. In adjusted models, maternal urine W and paternal urine U were associated with shorter GA, whereas blood Hg was associated with longer GA in both women and men. Higher maternal urine Cr was associated with lower delivery risk, although the risk was greater in association with higher paternal urine Ba, W, and U. Lower BW was associated with higher levels of paternal urine Cs, Cr, U, and Zn, even though paternal urine As and maternal blood Cd were linked to higher BW. BL was longer in conjunction with higher maternal and paternal blood Hg, yet BL was shorter for higher maternal urine W and paternal urine U. The latter was also linked to shorter HC. Of all analytes considered, the associations for urine W and U were most notable. Furthermore, boys appeared more vulnerable as maternal blood Pb was associated with a higher delivery risk and paternal urine Mo with shorter HC; yet we detected no effect for girls. Although no associations were detected for PI, lower values tended towards higher maternal exposure levels, potentially indicative of subtle yet undetected effects across pregnancy.

Elements in our sample (Tables 3 and ​and4)4) were low relative to U.S. population values for 2005 to 2010 (CDC, 2014a). U.S. women had modestly higher median blood Cd (0.273 μg/L) and Pb (0.73 μg/dL), as well as higher urine Sb (0.059 μg/L), Ba (1.56 μg/L), Cd (0.17 μg/L), Cs (4.65 μg/L), Co (0.45 μg/L), Pb (0.384 μg/L), Mo (48.48 μg/L), Tl (0.17 μg/L), W (0.08 μg/L), and U (0.006 μg/L) than study mothers. However, U.S. women had lower median total blood Hg (0.77 μg/L) and total urine As (7.60 μg/L). U.S. men had higher median blood Cd (0.23 μg/L) and Pb (1.17 μg/dL), as well as higher median urine Cd (0.153 μg/L), Co (0.364 μg/L), Pb (0.51 μg/L), Mo (54.3 μg/L), W (0.105 μg/L), and U (0.007 μg/L) compared to study fathers, although similar urine Sb (0.07 μg/L) and Cs (4.88 μg/L). Medians for U.S. men were also lower for total blood Hg (0.800 μg/L) and total urine As (8.93 μg/L).

Previous studies have reported associations between birth outcomes and maternal concentrations of metals and metalloids measured in blood or urine during pregnancy or at parturition, primarily among populations with exposures several-fold higher than we measured. Lower BW was reported in association with higher blood Pb in U.S. (Zhu et al., 2010) and Austrian (Gundacker et al., 2010) mothers, with higher maternal blood Cd in French smokers (Menai et al., 2012), and with higher blood Hg in GST1 null genotype Korean mothers (Lee et al., 2010). Lower BW and shorter BL was also associated with higher cord blood Pb in Taiwan (Tian et al., 2009). Decreased BW and HC was reported in association with higher urine Cd collected from Bangladeshi mothers (Kippler et al., 2012a), but only among girls, and with a non-linear ‘inverted U-shaped’ dose-response for fetal size assessed by ultrasound (Kippler et al., 2012b). Other investigators reported increased odds for high blood Cd among Saudi mothers with smaller crown-rump length deliveries (Al-Saleh et al., 2014), and higher cord blood Cd was associated with shorter BL among infants in a highly contaminated area of China, although there was no association for maternal blood Cd or for PD or BW (Zhang et al., 2004). Likewise, no associations were noted for GA or BL in a recent Polish study of maternal and cord blood Hg (Kozikowska et al., 2013). Whereas urine Cd was inversely associated with BW and urine Sn with HC among Japanese mothers, no associations were detected for BL or for levels of Sb, As, Be, Cu, Pb, Mo, Se, and Zn similar to ours (Shirai et al., 2010). A U.S. study more recently described a positive correlation between maternal blood Pb and PD among boys, also at levels similar to ours, although with no associations for BW, BL, or HC (Perkins et al., 2014).

Ours is the first report of effects on birth outcomes in association with background exposure to W and U. There exist very few experimental data to characterize the reproductive toxicity of W, and to our knowledge no observational studies have been conducted in humans (Keith et al., 2007). In contrast, U is well-recognized as a reproductive toxicant at high doses (Domingo, 2001). Recent animal evidence also suggests that U acts as an estrogen in vivo at low, environmentally relevant levels (Raymond-Whish et al., 2007). Early work in an occupational cohort indicated decreased SSR, although our data did not suggest an effect (Muller et al., 1967). No clinically significant reproductive effects were reported from a longitudinal study of n=74 depleted-U exposed U.S. Gulf War Veterans, although birth outcomes were not reported (Squibb and McDiarmid, 2006). If confirmed, our results merit further investigation to characterize the reproductive toxicity of W and U at background levels of exposure.

The positive associations we detected for GA and BL with maternal and paternal blood Hg, and BW and paternal urine As, were unexpected. However, the modestly elevated levels of blood Hg and urine As in our sample are likely to be indicative of seafood consumption (Marchiset-Ferlay et al., 2012; Mozaffarian and Rimm, 2006), and may thereby have served as markers for dietary exposure to long chain n-3 polyunsaturated fatty acids conferring benefits to fetal growth and development (Leventakou et al., 2014). We also detected a counterintuitive association for higher maternal blood Cd and increased BW. Increased fetal growth was likewise reported among Bangladeshi women with blood Cd below 1.5 μg/L (Kippler et al., 2012b). Still, this observation may reflect a chance occurrence given multiple comparisons and so we are circumspect in drawing conclusions. However, other results are consistent with the recent work assessing low dose gestational Pb (Perkins et al., 2014) and with long-recognized frailties among newborn males (Naeye et al., 1971).

Inconsistencies between our study results and those from prior studies are likely due to several factors, including differences in study design and in study populations. Exposure levels were generally lower than for prior reports and thus thresholds may not have been reached for previously reported birth outcome effects related to Cd, Pb, and Hg exposure. Thus, our findings may be useful to inform no observed adverse effect levels (NOAELs). Our multivariable models were also different from those used in prior studies as we adjusted for partner’s exposure and confounders to more realistically represent the couple-driven nature of reproduction. Although prior reports incorporated maternal-level confounding variables, we were able to assess the impact of paternal effects using our approach. In fact, we detected paternal effects with greater frequency than maternal during multivariable analysis. The sparse concordance between effects for partners underscores the importance of capturing exposure at the couple level. Furthermore, we used a biomarker based date of conception, whereas previous investigators employed bias-prone approaches relying on recall of the last menstrual period (Cooney et al., 2009).

Additional differences in data analysis strategies may account in part for discordance from prior study results. We included all births, without exclusion of PD or LBW, consistent with our a priori focus on ‘overall’ effects, although we may have missed subtle effects among clinical subgroups. Furthermore, we did not adjust for GA in assessing BW, BL, and HC, a common yet misdirected strategy likely to introduce bias when GA falls within causal pathways (Whitcomb et al., 2009). Finally, we collected biospecimens close to conception, although couples took up to 12 cycles for a pregnancy. The half-life for some elements is measured in hours-days (i.e., urine Sb, As, Ba, Cs, Co, Mo, Tl, W, and U) and is more vulnerable to exposure misclassification than those for which half-life is measured in weeks-months (i.e., blood Hg, blood and urine Cd and Pb) (CDC, 2009). However, prior studies indicate reasonable representation of As (intraclass correlation coefficient (ICC) = 0.49) (Kile et al., 2009) and Se (ICC = 0.77) (Longnecker et al., 1996) body burdens over time using single urine specimens. Furthermore, most of our participants (90%) conceived within six cycles of enrollment (Buck Louis et al., 2012). Still, exposure misclassification is possible, although very unlikely to have been differential according to outcomes given preconception biospecimen collection, and thus bias will have been towards the null hypothesis. Our a priori aim was to evaluate the impact of preconception couple-level exposures, the effects of which may differ from prenatal exposures.

The results of this exploratory study are limited by several factors. Despite a moderate sample size of 235 couples, we included a number of covariates in regression models which resulted in some sparse strata and imprecise estimates, in particular for interactions in which we detected small differences of questionable clinical relevance. For some elements (e.g. W), a substantial proportion of values fell below detection limits and so the results will require confirmation. For the essential elements, which are normally under homeostatic control in the body, serum and blood (i.e., Cu, Mn, Se, Zn), or plasma (i.e., Cr) measurements are considered preferable to urine, certainly for assessing nutritional status (Arnaud et al., 2008; CDC, 2009; Paustenbach et al., 1997; Sunderman, 1993). While other samples can be used to assess long term or historical exposure, they may be compromised by exogenous contamination (i.e., hair, nails) or they are difficult to obtain (i.e., bone). For these reasons, spot urine samples are widely used for exposure assessment studies, yet we would concede that they may not represent the best biomarker of exposure for all of the elements measured. Nonetheless, we used a highly sensitive multi-element ICP-MS method for urine that was optimized for biomonitoring studies as described previously (Pollack et al., 2013). Thus, we report a majority of detected values for Cu, Se, and Zn in urine, with the exception of Mn, which we did not consider further. We generated regression models based on individual elements to identify predictors of interest for future confirmation and thus unmitigated confounding by excluded elements might account in part for unexpected results. We were also unable to accommodate additional reproductive toxicants into the analysis that might have confounded associations. Still our limited sample size is likely to have reduced study power and so we may have missed subtle associations. Additionally, we conducted many independent statistical tests, without accommodating the likelihood for type-1 error inflation. However, our intent was to maximize the sensitivity for detecting modest associations to inform future investigations (Goldberg and Silbergeld, 2011).

5. CONCLUSIONS

To the best of our knowledge, this is the first report to describe preconception parental element exposures and birth outcomes. Most consistently, we detected associations between maternal urine W and paternal urine U and birth outcomes, and also unique effects for maternal blood Pb and paternal urine U among boys. Though limited by several factors, ours is the largest multi-element investigation of prospective couple-level trace exposures and birth outcomes to date; the novel observations for W and U merit further investigation. It is critical to identify potentially modifiable risk factors to inform intervention strategies and to reduce risks for adverse birth outcomes.

Supplementary Material

Acknowledgments

Footnotes

References