A study by UK doctors suggests zinc supplements may cause poor mental development in infants.

Many undernourished women in developing countries are advised to take extra zinc in an effort to boost the weight of their unborn child and improve its immune system.

If women are eating a well-balanced diet, which in terms of zinc includes wholemeal bread and cereals, then they really have nothing to worry about

Mary Nolan, National Childbirth Trust

However, doctors at the Institute of Child Health in London say their findings raised questions about that policy.

Professor Sally Grantham McGregor and colleagues assessed the mental development of 168 infants in Bangladesh.

Their mothers had been divided into two groups while pregnant. The first group had received 30mg of zinc every day during pregnancy, while the second group received a placebo or dummy pill.

The doctors found that infants whose mothers had been in the second group had higher scores for both mental and psychomotor development or the ability to control movement.

Fresh approach

Professor Grantham McGregor acknowledged that her findings would "complicate" existing policies.

But she said they highlighted the need for a more comprehensive approach to improving the health of pregnant women and their children in the developing world.

Writing in The Lancet, she said: "Undernourished pregnant women obviously require more than zinc alone.

"The next step would be to examine the effect of more comprehensive supplementations to improve maternal nutritional status during pregnancy on a broad range of outcomes including infants' development."

Mary Nolan of the National Childbirth Trust said few women in western countries need extra zinc during pregnancy.

"If women are eating a well-balanced diet, which in terms of zinc includes wholemeal bread and cereals, then they really have nothing to worry about. They will be getting what their body needs."

Speaking to BBC News Online, she added that pregnant women should always consult a health professional before deciding to take extra vitamins.