We conducted a prospective cohort study to examine the effects of the internal THM dose totalled over the first trimester of pregnancy, and separately for each of the first 3 months of pregnancy, on congenital anomalies. Our study estimated the internal dose using information on individual women's water use. Individual exposure showed similar associations with the risk of congenital anomalies, whether it was analysed as a continuous variable or categorised into tertiles. We found little indication of a dose-response relationship between exposure to TTHM and CH and congenital heart and urogenital anomalies. These results were similar to the results obtained when comparing exposure to TTHM of high and low-level sites, in which high exposure was associated with an increased risk of heart and urogenital anomalies. When analysed as continuous variables, TTHM and CH exposure in the first trimester of pregnancy showed slightly elevated, but statistically non-significant, increases in the risk of congenital heart, urogenital and musculoskeletal anomalies. The relationship was stronger for brominated THM. For the congenital heart anomalies, a dose-response relationship was evident for BDCM exposures in the first month of pregnancy; χ2 test for trend p was 0.024. ORs increased by 70% (OR 1.70, 95% CI 1.06 to 2.66) for every 0.1 μg/d increase in the internal dose of BDCM and by 26% (OR 1.26, 95% CI 1.01 to 1.54) for every 0.01 μg/d increase in the internal dose of DBCM.
There were statistically significant dose-response trends across the DBCM exposure categories for musculoskeletal anomalies (p=0.024); and across the BDCM exposure categories for urogenital anomalies (p=0.039). In the present study, the internal dose studied as a continuous variable more often revealed statistically significant association than in categorical analysis unadjusted χ2 test for trend.
Etiologic studies suggest that major structural anomalies occur within the first trimester of pregnancy. We found that congenital anomalies were associated with the internal THM dose during the first trimester of pregnancy, particularly in the first 2 months of pregnancy. These results may be related to the limited variation of the month-specific internal THM dose. In our analyses, it was difficult to evaluate the independent effects of exposures by month because they were highly correlated.
Reconciling our results with previous findings is not straightforward because of substantial differences in THM levels, individual THM constituents in drinking water, measurement and classification of individual exposures, variation of exposure over the months of pregnancy and the extent of controlling for confounders. In addition, our sample did not capture stillbirths and pregnancy terminations due to congenital anomalies diagnosed prenatally.
The specific mechanisms for the effects of THM on the risk of birth anomalies remain unknown. There is evidence that the metabolism and toxicity of different DBP species varies.23
In general, the brominated DBPs are more genotoxic and carcinogenic than the chlorinated compounds, and iodinated DBPs are the most genotoxic.25
Several mechanisms for the effects of THM have been suggested, including genotoxicity, oxidative stress, disruption of folate metabolism, lowering of testosterone levels and disruption of the synthesis and/or secretion of placental syncytiotrophoblast-derived chorionic gonadotropin.26
Because the brominated THMs are structurally similar, and because there is evidence for common pathways of bioactivation, findings27
support the idea that glutathione (GSH) conjugation of tribromomethane may lead to the formation of DNA-reactive metabolites in the liver and, more likely, in the colons of rodents and humans.
Brominated THM is thought to present a greater health risk than CH, primarily because of differences in their metabolism and toxicokinetics.28
In addition, BDCM can disrupt syncytiotrophoblast formation and inhibit chorionic gonadotrophin secretion in vitro.29
This finding implies that the placenta is a likely target of BDCM toxicity in humans; thus, BDCM may have teratogenic effects on the fetus. An alternative explanation is that THM may lead to birth defects via genetic damage to maternal gametes.30
This damage may result in chromosomal abnormalities, enzymatic malfunction and disruption of cellular membranes, all of which could influence the formation of anomalies.
Only a few studies have investigated associations between BDCM levels in drinking water and congenital anomalies. A study in southeast England31
that examined the risk of hypospadias and exposure to THM through water consumption and use concluded that ingestion of more than 6 mg/d of BDCM was associated with the risk of hypospadias (OR 1.65, 95% CI 1.02 to 2.69). A population-based Canadian study reported a statistically significant association between BDCM and neural tube defects,9
whereas a study in both Canada and the USA found a negative association with neural tube defects and cleft lip and palate.13
A study in England and Wales16
reported that high total brominated THM exposures in the first trimester of pregnancy were not associated with significant excess risk of congenital anomalies. An Australian study reported a statistically significant increased risk of any congenital anomalies (OR 1.22, 95% CI 1.01 to 1.48) and of cardiac anomalies (OR 1.62, 95% CI 1.04 to 2.51) among women exposed to high levels of TTHM in drinking water with high proportions of brominated THM (on average, 92%).32
These results are consistent with our data in which the highest risk for congenital anomalies comes from brominated THM. To our knowledge, no previous studies have shown an association between the internal THM dose during pregnancy and the risk of congenital heart anomalies.
The strengths of our study include the population-based cohort design, the assessment of THM exposure during pregnancy, and the control for the effects of residential mobility by restricting the study to women who did not change residence during their pregnancy. This study also used advanced methods to calculate individual internal THM exposure during pregnancy based on residential THM levels and water use behaviours. Each subject's exposure was estimated as a daily internal dose of the THM constituents (μg/d). Exposures were analysed using both continuous and categorical variables. An additional strength of our study is that pregnant women were prospectively followed, which permitted collection of self-reported data on potential confounding factors, decreased exposure misclassification errors, and improved identification of congenital anomalies.
We acknowledge several limitations in this study. We did not gather information on water usage habits during the first trimester of pregnancy. Instead, women were interviewed during the third pregnancy trimester before delivery, which may have affected the estimation of THM uptake and may have led to exposure classification errors. However, water consumption habits and unmeasured confounders were likely to vary independently of the three THM exposure categories, and should not confound the relationships we observed. Misclassification of congenital anomalies was unlikely in this prospective study, as the presence of major congenital anomalies is recorded in the birth register and generally considered reliable. A study of congenital heart defect diagnoses in the infant population of Kaunas revealed that, over 7 years, up to 93.9% of congenital heart anomalies were diagnosed in delivery units.33
We have no possibility of studying the diagnostics of non-syndromic forms of the kidney and urinary tract anomalies. However, the analysis of anomalies diagnosed before the infant was discharged from the hospital should not bias study results, as the completeness of reporting is unrelated to the exposure of interest. In addition, the classification of congenital anomalies in our study was independent of exposure assessment.
Due to the lack of information regarding the validity of the internal dose assessment models used in our study, it is possible that the effect estimates we observed may be biased because of non-differential misclassification of the internal dose.
Our study findings show that higher levels of the brominated THM internal dose during the first trimester of pregnancy may be associated with an increased risk of congenital heart and musculoskeletal anomalies. Recently reported DBP toxicity from samples of the Kaunas HIWATE programme sites revealed that the number of identified DBPs, the level of DBPs, the cytotoxic potency and the genotoxic potency were all greater for sites with ‘high level’ THM relative to ‘low level’ THM.34
There was a clear difference in the genotoxic responses of the Kaunas ‘high level’ versus ‘low level’ THM site samples. These data suggest that the results of our epidemiological study are consistent with results from analytical chemistry and in vitro toxicology studies. However, the association between the internal THM dose and the risk of congenital anomalies observed in our study may be due to DBPs that were not studied, or to other toxic water contaminants, or occurred by chance.
Further studies are required to clarify the association of individual THM internal doses and congenital anomalies. Our results are preliminary and need to be confirmed in a larger sample with more variability in THM concentrations and internal THM doses. Investigations of drinking water DBPs that integrate quantitative toxicological data with analytical chemistry and human epidemiologic data to look at gene-environment interactions are one possibility. Given the controversy surrounding the association of THM levels in drinking water and adverse pregnancy outcomes, especially regarding congenital anomalies, a precautionary approach to brominated THM exposure during pregnancy is justified.