File photo: In fact mothers who had taken the supplements were more likely to report that their children were badly behaved than those given the dummy pills.

Pregnancy supplements taken by thousands of women to boost the IQ of their babies are a waste of money, researchers claim.

Omega 3 tablets made no difference to children’s language or developmental skills, a seven-year study of 543 mothers found.

Researchers looked at supplements containing an omega 3 fatty acid found in fish such as salmon, known as docosahexaenoic acid or DHA.

But research published in the Journal of the American Medical Association concluded there was "little evidence of benefit". Academics from the South Australian Health and Medical Research Institute in Adelaide looked at 543 pregnant women.

Half were given daily supplements containing DHA for the last half of their pregnancy while the remainder took dummy pills.

Tests then monitored the children’s development when they were 18 months, four years old and seven years old. No difference was found in the two groups’ intelligence, language skills or IQ.

In fact mothers who had taken the supplements were more likely to report that their children were badly behaved than those given the dummy pills.

The study said: "Direct assessments consistently demonstrated no significant differences in language, academic abilities, or executive functioning. The sale of prenatal supplements with DHA continues to increase, despite little evidence of benefit to offspring neurodevelopment."

These latest findings back up research last July which found no evidence that pregnancy supplements improved the health of mothers or their babies.

Advice is for pregnant women to take folic acid in the first trimester and vitamin D for the whole pregnancy.

There is no recommendation for omega 3 supplements.

Dr Virginia Beckett, from the Royal College of Obstetricians & Gynaecologists, said: "While most vitamins and minerals come from our diet, it is common for people in the UK to be low in vitamin D and folic acid, which are important in pregnancy."

She said the college agreed with the study’s recommendation for mothers to take supplements of those two nutrients, alongside a healthy diet and exercise. She added this was "enough to give their child the best start in life".

A Health Food Manufacturers’ Association spokesperson, representing the supplements industry, said: "This single study fails to reflect the huge body of positive evidence showing the importance of DHA for cognitive function."

The spokesperson added that authors of several studies in 2013 concluded that supplements such as DHA did raise the intelligence of young children.