Although some studies have suggested that certain chemical byproducts in tap water raise a woman's risk of miscarriage, new research suggests that the threat is small, if it exists at all.

The chemicals in question are byproducts of the chlorination process used to kill disease-causing pathogens in the drinking-water supply. Some of these byproducts, including a group of chemicals called trihalomethanes, have been shown to cause cancer and reproductive problems in lab animals exposed to high doses.

In addition, some population studies have found an association between low-level exposure to these chemicals and a higher risk of miscarriage and poor fetal growth. Other studies, however, have found no such relationships.

In this latest study, which followed more than 3,100 pregnant women who got their drinking water from one of three water systems, researchers found no association between overall exposure to trihalomethanes and the risk of miscarriage.

There was, however, an increased risk of miscarriage and impaired fetal growth among women with the highest exposure to one type of trihalomethane called bromodichloromethane (BDCM).

Still, the findings should be reassuring to pregnant women worried about the safety of their tap water, according to lead researcher Dr. David A. Savitz of the University of North Carolina School of Public Health in Chapel Hill .

While he told Reuters Health he did not want to downplay the risk that may be posed by BDCMs, Savitz said the overall findings suggest that if there is a pregnancy threat, it is likely to be slight.

A report on the study was published online by the American Water Works Association Research Foundation, an international, nonprofit research organization that partially financed the study. The US Environmental Protection Agency (EPA) also provided funding.

Water-disinfection byproducts form when a disinfectant, often chlorine, reacts with certain chemicals or organic matter, such as decaying vegetation, naturally present in the drinking-water source. The EPA has set an upper limit for some of these byproducts, including trihalomethanes -- though the EPA says there is insufficient evidence to link the chemicals to reproductive health effects.

Exposure to disinfectants

The new study findings conflict with those from some previous studies, including a recent one, in which California researchers found an association between trihalomethane exposure and miscarriage risk.

Savitz said his team's study was more rigorous. The researchers chose three sites with varying levels of disinfection byproducts in the water, including one with very low levels of all byproducts and two others with moderate levels. They took weekly samples from each source to monitor chemical levels, and women in the study reported how much water they drank, filtered and unfiltered, as well as their bathing habits.

The researchers found no relationship between overall trihalomethane exposure and miscarriage risk -- women with the greatest exposure had a lower rate of premature delivery.

Given that three quarters of Americans have chronic low-level exposure to disinfection byproducts, there is a need for ongoing research into the potential health effects, according to Savitz.