Estimating Effects of Arsenic Exposure During Pregnancy on Perinatal Outcomes in a Bangladeshi Cohort

Abstract

Background: The relationship between arsenic and birth weight is not well understood. The objective was to evaluate the causal relationship between prenatal arsenic exposure and birth weight considering the potential mediation effects of gestational age and maternal weight gain during pregnancy using structural equation models.

Methods: A prospectively enrolled cohort of pregnant women was recruited in Bangladesh from 2008 to 2011. Arsenic was measured in personal drinking water at the time of enrollment (gestational age <16 weeks, N = 1,140) and in toenails collected ≤1 month postpartum (N = 624) using inductively coupled plasma mass spectrometry. Structural equation models estimated the direct and indirect effects of arsenic on birth weight with gestational age and maternal weight gain considered as mediating variables.

Results: Every unit increase in natural log water arsenic was indirectly associated with decreased birth weight (β = -19.17 g, 95% confidence interval [CI]: -24.64, -13.69) after adjusting for other risk factors. This association was mediated entirely through gestational age (β = -17.37 g, 95% CI: -22.77, -11.98) and maternal weight gain during pregnancy (β = -1.80 g, 95% CI: -3.72, 0.13). When exposure was modeled using toenail arsenic concentrations, similar results were observed. Every increase in natural log toenail arsenic was indirectly associated with decreased birth weight (β = -15.72 g, 95% CI: -24.52, -6.91) which was mediated through gestational age (β = -13.59 g, 95% CI: -22.10, -5.07) and maternal weight gain during pregnancy (β = -2.13 g, 95% CI: -5.24, 0.96).

Conclusion: Arsenic exposure during pregnancy was associated with lower birth weight. The effect of arsenic on birth weight appears to be mediated mainly through decreasing gestational age and to a lesser extent by lower maternal weight gain during pregnancy.

FIGURE 1.

Effect sizes for the initial conceptual SEM model are presented. This model hypothesized that natural log-transformed arsenic water (A) or toenail (B) directly effects (c′) birth weight (g) after adjusting for gestational age (weeks) and maternal weight gain (kg). The partial effects of arsenic on gestational age (a₁) and maternal weight gain (a₂), as well as, the partial effects of gestational age (b₁) and maternal weight gain (b₂) on birth weight are presented.

FIGURE 2.

Final SEM models for the indirect effect of log-transformed measured in drinking water arsenic (A) or toenail arsenic (B) on birth weight (g) that is completely mediated through gestational age (weeks) and maternal weight gain (kg) adjusting for other risk factors.