An Australian study suggests there is no real need for pregnant women to take fish oil supplements in a bid to make their child smarter.

Taking fish oil supplements during pregnancy will not guarantee the child will go on to be a Rhodes scholar later in life.

According to a new Australian study taking DHA (docosahexaenoic acid) supplements - an omega-3 fatty acid found in fish oils - while pregnant does not lead to children with higher IQs.

Scientists at the South Australian Health and Medical Research Institute in Adelaide compared women given a daily dose of 800mg of docosaheaenoic acid (DHA), an omega-3 fatty acid found in fatty fish, during the second half of their pregnancies with those given a placebo.

The randomised control trial found no improvements in cognitive, language, and motor development at 18 months after birth, according to the findings published in the JAMA journal.

The researchers also didn't find any improvements in general intelligence, language, and executive functioning at age four and again at age seven.

In fact, kids of mums given the supplements tended to be "naughtier" and less mentally capable in terms of executive functioning, although the negative effects were small, said the researchers.

Of the 543 children who participated in the seven-year follow-up, the average IQ of the DHA and control groups did not differ, with the groups scoring 98.31and 97.32 points respectively.

Direct assessments consistently demonstrated no significant differences in language, academic abilities, or executive functioning, although perceptual reasoning was slightly higher in the DHA group, the authors noted.

Co-author of the study Dr Jacqueline Gould says there had been a few studies in the past that had shown fish oil might benefit the brain of the unborn baby however the evidence wasn't clear.

Dr Gould says while fish oil supplements are quite popular for pregnant women to take they've found there's no need for them.

"The brain is quite rich in DHA, which is the main fatty acid in fish oil, and DHA is accumulated in the brain rapidly during that last trimester of pregnancy," Dr Gould told AAP.

"If (women) have got a typical pregnancy with a healthy varied diet then their baby's brain is not going to get any extra benefit from taking fish oil supplements."