According to the researchers, oily fish is not recommended to pregnant women as it contains mercury, a substance which is bad for health in large doses.
The findings were recently published in the New Scientist magazine.
The American scientists examined nearly 1,000 pregnant women from 52 prenatal clinics. Researchers determined the mercury level participants at the study had in their hair, at a beginning of the study and in the moment each woman gave birth to her baby.
Figures revealed that those giving birth prematurely had the mercury level in their hair doubled.
Anyway, researchers are hoping to develop further studies as only 44 of the 1,024 pregnant women had their babies earlier than it was planned.
"Additional studies are needed to see if these findings will be replicated in other settings," said Dr Xue, the lead researcher.
Moreover, it remains uncertain if the mercury level was increased because of eating oily fish. There are speculations that other factors raised the mercury level.
However experts warn that although oily fish like sardines, salmon or mackerel should be avoided, pregnant women are forbidden to cut fish from their diet.
"The messages are really very conflicting because fish is both a benefit and a potential source of hazard," explained Dr Xue.
According to the researchers, women who are pregnant, who are breastfeeding or are trying to conceive a child should have fish meals twice a week.
Healthy adults, omitting the above mentioned categories, should eat fish four times a week.
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