Fish oil supplements taken by pregnant women do not boost the baby’s mental development nor does it help prevent postpartum depression according to an Australian study.

Many mothers to be are encouraged to take fish oil supplements during pregnancy because the key fish oil ingredient, docosahexaenoic acid, is believed to be beneficial.

The research team from the Women's and Children's Hospital in North Adelaide analyzed more than 2,000 women who had taken daily supplements of either fish oil with DHA or a placebo (vegetable oil) during the second half of their pregnancy.

Researchers found overall cognitive scores to be nearly identical and language scores tended to be lower in children exposed to DHA-rich fish oil during gestation than scores in the control group.

The study published in the Journal of the American Medical Association showed there was also no significant difference in the prevalence of depressive symptoms in the first six months postpartum between women who took the fish oil pills and those who didn't.

The new report suggests DHA supplementation in pregnancy reduces the likelihood of giving birth before 34 weeks of pregnancy.

"Before DHA supplementation in pregnancy becomes widespread, it is important to know not only if there are benefits, but also of any risks for either the mother or child," said researcher Maria Makrides in a statement.

The only side effect found in using the supplement is belching.