Background Neural tube defects (NTDs) are a group of severe birth defects caused by both genetic and environmental factors, and in addition to maternal folate deficiency, other risk factors remain to be explored. Previous studies have found that high levels of methylmercury (MeHg) and total mercury (T-Hg) in placental tissues increase the risk of NTDs. In a case-control study evaluating the association between prenatal mercury exposure and the risk of NTDs by detecting the concentrations of MeHg and T-Hg in umbilical cord tissues, it is necessary to determine whether the MeHg or T-Hg in umbilical cord tissues is affected by gestational age because of the significant difference in gestational age between two groups. But to our knowledge, only one related study with a small sample size is available.
Objective This study is to compare the concentrations of MeHg and T-Hg in umbilical cord tissues of fetuses with NTDs at different gestational ages, evaluate relationships of gestational age with MeHg or T-Hg, and describe the metabolism and distribution characteristics of MeHg and T-Hg in fetuses.
Methods Automated methylmercury system and manual total mercury system were used to measured MeHg and T-Hg in frozendried umbilical cord tissue samples of 177 fetuses with NTDs who were diagnosed prenatally or at birth. Median (M) and interquartile range (P25-P75) were used to describe the average level and dispersion of MeHg and T-Hg (ng/g, in terms of dry weight). According to gestational age, the samples were divided into four groups (13-23, 24-29, 30-35, and 36-43 weeks old) to compare MeHg and T-Hg concentrations. Spearman rank correlation was used to evaluate the correlations of gestational age with MeHg and T-Hg concentrations. Multivariate linear regression was used to evaluate the relationship between gestational age and MeHg concentrations after natural log transformation, adjusting for confounders including maternal age, education level, occupation, year of induction labor or delivery, and drinking alcohol and consumption of seafood during pregnancy.
Results The MeHg and T-Hg concentrations in M (P25-P75) were 1.99 (1.30-3.20) ng/g and 5.85 (4.11-8.48) ng/g, respectively. The MeHg concentrations among the four groups of different gestational ages were significantly different (P=0.015), and raised from 1.73ng/g to 3.04ng/g with the increasing of gestational age. Gestational age was positively correlated with MeHg concentration in umbilical cord (rs=0.188, P=0.012), and after adjusting for maternal age, education level, occupation, year of induction labor or delivery, and drinking alcohol and consumption of seafood during pregnancy, MeHg concentration in umbilical cord still increased with older gestational age (b=0.024, P < 0.001). The accumulation trend of MeHg consistently existed in male or female NTDs fetuses and spina bifida subtype. No significant difference in T-Hg concentration was found among the four groups of different gestational ages.
Conclusion MeHg might accumulate in the umbilical cord tissues of NTDs fetuses in a gestational age-dependent manner, but T-Hg concentration does not follow such pattern.
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