SCIENTISTS are calling for cancer-causing chemicals in swimming pools to be cut amid fears they are linked to birth defects.
Research shows that swimming pools could be a "major pathway" for the uptake of by-products of chlorination among pregnant women.
Swimming is widely recommended as a safe form of exercise for expectant mothers and swimming pools and leisure centres across the country offer special pre-natal sessions.
But the warning comes amid "rising concern" over a possible link between such chemicals and miscarriage, according to Dr Mark Nieuwenhuijsen, of London's Imperial College of Science and Technology and Medicine.
Levels of the by-products are relatively high in indoor UK swimming pools, research published in the journal Occupational and Environmental Medicine found.
When chlorine is added to water it reacts with organic matter, such as skin cells and body care products, to form disinfection by-products (DBPs).
The most common of these are trihalomethanes (THMs), volatile compounds that can vaporise from water into the atmosphere and the most common of these is chloroform.
Miscarriages, low birth weight and some birth defects have been associated with THMs, but current evidence appears to be inconsistent and inconclusive, the researchers said.
Dr Nieuwenhuijsen said: "For many decades, chlorination has been used as a major disinfectant process for public drinking and swimming pool water in many countries.
"However, there has been rising concern over the possible link between DBPs and adverse reproductive outcomes."
He said that, as a precautionary measure, THM levels "should be reduced as far as possible in swimming pools while maintaining effective control against waterborne microbiological disease".
Researchers took 44 water samples from eight indoor swimming pools and compared them with tap water. The samples were taken at different times over three weeks during the summer of 2000.
Total organic matter was three times higher in swimming pool water than tap water, and chloroform was more than 20 times higher.
Chloroform has been classed as a potential cancer-causing agent.
They suggest that there are several routes of entry for swimmers - through the skin, swallowing water, and breathing it in off the surface of the water as it evaporates.
They conclude that pregnant women, who go swimming often, may be at risk. Further work should be carried out to assess the importance of the relatively high concentrations of THMs on potential health risks, the researchers insisted.