Pregnant women who swim regularly could be at increased risk of miscarriage and birth defects because of the high chemical content of public pools.

The process of chlorination, which for decades has been used to disinfect pools, results in potentially carcinogenic by-products, the most common of which is chloroform.

Scientists who tested eight indoor pools in London found "relatively high" concentrations of the disinfectant by-products, which have been linked to reproductive problems. In a study, published yesterday in Occupational and Environmental Medicine, they called for levels of trihalomethanes (THMs) in pools to be cut as much as possible.

Chloroform has been classed as a potential cancer-causing agent and some research has suggested that THMs may harm the foetus and could trigger miscarriages.

While the evidence seemed to be "inconsistent and inconclusive", the researchers said there was "rising concern" that exposure to the chemicals was linked to miscarriages, neurological defects and low birth weight babies.

The main author of the study, Dr Mark Nieuwenhuijsen, of Imperial College, London, did not recommend that women should stop swimming during pregnancy. But he said: "Swimming pools could be a major pathway for the uptake of trihalomethanes among pregnant women who go swimming often."

When chlorine is added to water it reacts with organic matter, such as skin cells and body care products, to form disinfection by-products, including THMs which can vaporise from the water into the atmosphere.

The researchers found that total organic matter was three times higher in swimming pool water than tap water, and chloroform levels were 20 times higher. THM levels varied from day to day, according to the numbers of swimmers, but they were higher, overall, than the levels reported by other European studies. THM concentrations generally increased with warmer water and more people in the pool.

The researchers did not measure uptake of THMs. But previous research has indicated that a one-hour swim would provide a chloroform dose about 141 times higher than a 10-minute shower. Other studies have recorded "considerable uptake" of chloroform during swimming.

The International Agency for Research on Cancer has listed chloroform as a potential carcinogen. Swimmers are likely to absorb THMs through the skin and by inhalation from the air above the pool water surface, although some will be swallowed.

Dr Nieuwenhuijsen said it was essential to gain a better understanding of the potential harm caused by THMs, possibly by large-scale population studies. He concluded: "Trihalomethane concentrations should be reduced as far as possible in swimming pools while maintaining effective control against waterborne microbiological disease".

* Exposure to sunlight could help reduce death rates from breast, ovarian, colon and prostate cancer, according to another study in the journal. While death rates from skin cancer were higher in very sunny climates, an 11-year study in America showed deaths from the four other cancers were significantly lower.

Researchers from the National Cancer Institute in Bethesda, Maryland found that the US regions of greatest sunlight were associated with a reduced risk of the cancers.