Expectant women who drink the water, swim in it, take a bath or shower in it, or even stand close to a boiling kettle, could double their chances of giving birth to a baby with heart problems, a cleft palate or major brain defects.
Chlorine contains chemicals called trihalomethanes, or THMs, and these increase the risk of holes in the heart, cleft palate and anencephalus, where parts of the brain, skull and scalp fail to fully develop.
Researchers from the University of Birmingham estimate that one in six households in Britain - around 4 million homes - could be supplied with water with dangerously high levels of chlorine.
The latest study is just one of many that have alerted water authorities to the dangers of over-chlorination, but so far none has reduced the levels.
(Source: Environmental Health, 2008; 7: 23).