Eating too much oily fish during pregnancy may increase the risk of delivering the baby too early, scientists believe.
Too little or too much fish in the diet can be harmful
The researchers told New Scientist magazine the harm is probably caused by high mercury levels in oily fish such as mackerel, salmon and sardines.
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, it recommends.
Girls, 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.
Source: Food Standards Agency
Oily fish are high in beneficial fats such as omega 3.
Studies show eating enough fish can boost the birth weight and brain power of babies and help prevent premature labour in pregnant women.
The latest work in New Scientist, also published in the journal Environmental Health Perspectives, looked at 1,024 pregnant women living in Michigan, the US.
Dr Fei Xue and colleagues measured the amount of mercury these women 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.
On the whole, these women also tended to eat more oily fish, and particularly canned fish.
Only 44 of the women gave birth prematurely, however, 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 is also possible that the women could have been exposed to mercury from other sources too, they said.
Dr Xue said until the risks become clearer, women could take fish oil supplements instead.
A spokeswoman from the British Nutrition Foundation said: "If pregnant women do decide to take supplements, it is important to read the label and check that the supplement does not contain high amounts of vitamin A (retinol). Too much retinol can be toxic to the developing baby."