Exposure to high levels of a by-product of water chlorination - chemicals known as trihalomethanes (THMs) - doubled the risk of holes in the heart, cleft palate and anencephalus, which results in the absence of a major portion of the brain, skull and scalp.
What is the research?
The Taiwanese and UK research team analysed birth records for 396,049 babies born in five regions in Taiwan, between 2001-2003, for common birth defects.
Birth records included information on any defects diagnosed between 20 weeks of pregnancy and seven days after birth, as well as data on the mother including any pre-existing medical conditions. A total of 2,146 babies were reported to have birth defects.
Based on where the mother lived, researchers estimated her level of exposure to THMs in the water supply during pregnancy using records of water quality obtained from the water treatment plants.
However, the study did not follow the women during pregnancy and assess the water they drank, so the researchers lacked exact information about what each woman drank during her pregnancy or about other possible exposures to THMs by swimming or bathing.
Researchers also searched an online database for other studies published between 1996 and 2007 that assessed the effect of the by-products of chlorination on birth defects.
They then pooled the results of their study with those of the five published studies they identified.
Overall, the results showed no association between exposure to THMs and the risk of birth defects in general.
Looking at specific defects they found that women who had been exposed to high levels of THM over 20 micrograms/litre were 1.81 times more likely to have babies born with ventricular septal defects and 1.96 times more likely to have babies with anencephalus than women who had been exposed to low levels of THM below 5 micrograms/litre.
Babies were also 1.56 times more likely to develop a cleft palate if the mother had been exposed to high THM levels.
Researchers propose that THMs may lead to birth defects via genetic damage to maternal gametes.
What do the researchers say?
Lead researcher Professor Jouni Jaakkola, from the institute of occupational and environmental medicine at the University of Birmingham, said: 'Findings suggest exposure to chlorination by-products may be responsible for some specific and common defects.
'But the biological mechanism for how these disinfection by-products may cause defects are still unknown.'
More research needs to be carried out to determine these side-effects, he said.
What do other researchers say?
Phillip Mills, deputy chief executive at the water industry body Water UK, said that a number of other studies looking at chlorinated water, including one involving 2.5 million people in England and Wales, had not found any link to birth defects.
'Chlorination is used across the UK as a method of disinfecting water in line with WHO guidelines.
'If you were to stop chlorinating water then it would lead to a rise in water-borne diseases.'
Benefits of chlorinated water outweigh the risks, he added.
- Babies born in areas where drinking water is heavily chlorinated could be at double the risk of defects.
- The majority of studies show no link between tap water and birth defects.
- Benefits of chlorinated tap water far outweigh the risks.
- In England levels of THMs in tap water range from 5-10 micrograms/litre.
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