According to scientists, excessive consumption of oily fish during pregnancy may increase the risk of premature delivery.
The researchers suggested that the high mercury levels in oily fish like mackerel, salmon and sardines could be responsible for this effect. However, experts say that it is essential for everybody including pregnant women to eat sufficient fish to stay healthy.
According to the Food Standards Agency, pregnant women should eat fish twice a week. However, it recommends them to avoid fish like shark, marlin and swordfish due to their high content of mercury and other pollutants.
Two portions of oily fish per week is recommended for women trying to conceive and breast feeding women. Others including men and boys can have up to four portions a week. Oily fish are rich in omega-3 fatty acid.
Studies have proved that birth weight and brainpower of babies can be enhanced by consumption of sufficient fish. It also prevents premature labour in pregnant women.
A recent study published in the journal Environmental Health Perspectives, examined 1,024 pregnant women living in Michigan, the US.
Dr Fei Xue and colleagues determined the amount of mercury in the hair of these women and did a comparative analysis with the date of delivery.
The chances of having twofold the average mercury level in hair was 3 times higher in women who gave birth more than two weeks early. In general, these women seemed to consume more oily fish, especially canned fish.
However, premature birth was observed in only 44 women. Researchers said that more work was required to confirm their findings. They also said that some amount of inaccuracy might also be there as the amount of fish eaten was recorded based on the recollection of those women.
Dr Xue said: "It is also possible that the women could have been exposed to mercury from other sources too. 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."