NEW YORK: Researchers in the US have warned pregnant women to limit their fish intake citing high mercury levels in oily fish.

The high mercury levels in fish such as mackerel, salmon and sardines could cause them harm, reported the online edition of BBC News quoting a report published in the New Scientist magazine.

Dr. Fei Xue and colleagues looked at 1,024 pregnant women living in Michigan and measured the amount of mercury they had in their hair and compared this with the date that the women delivered their babies.

The women who gave birth more than two weeks early were three times as likely to have double the average mercury level in their hair samples, the researchers said. On the whole, these women also tended to eat more oily fish, particularly canned fish.

Only 44 of the women, however, gave birth prematurely, and the researchers said more work was needed to corroborate their findings.

They also pointed out that the women were asked to recall how much fish they had eaten, which might be inaccurate. It was also possible that the women could have been exposed to mercury from other sources too, they said.

Xue said until the risks become clearer, women could take fish oil supplements instead.

But experts warn it is important for pregnant women, and indeed everyone, to eat enough fish to keep healthy. Pregnant women should eat fish twice a week, says the Food Standards Agency.

But they should avoid shark, marlin and swordfish because these fish are particularly high in mercury and other pollutants.

Women who are breastfeeding and those trying for a baby should also eat two portions of oily fish per week. Other women, and men and boys, can eat up to four portions per week. One portion is about 140g of fish - one tuna steak, for example.