This content has not been reviewed within the past year and may not represent WebMD's most up-to-date information. To find the most current information, please enter your topic of interest into our search box. April 8, -- For decades, chlorine has been used to disinfect public drinking water and swimming pools. Now, British researchers say that a chemical by-product of chlorine -- found in very high levels in swimming pools -- may be hazardous for pregnant women and their unborn babies. In the study, levels of this by-product -- called trihalomethanes or THMs -- were considerably higher in swimming pools than in tap water, writes study author Mark J.