Women who are strict vegetarians or vegans may be a greater risk of having a child with birth defects because they are likely to be deficient in vitamin B12, researchers warned.

Research carried out in Ireland has found that women with low levels of B12, found in meat, eggs and milk, when they conceive are at greater risk of having a child with neural tube defects.

These conditions include spina bifida, which causes partial paralysis, and anencephaly where the brain does not develop and is normally fatal shortly after birth.

Women who may become pregnant or who are pregnant are advised to take folic acid supplements because it is known that the vitamin folate protects against these defects and it has been suggested that taking vitamin B12 may reduce the risk further.

A team from the National Institutes of Health, Trinity College Dublin, and the Health Research Board of Ireland found women with low levels of B12 were 2.5 to three times more likely to have a child with a neural tube defect while those classed as deficient in B12 were five times more likely to have a child with a defect.

The study is published in the journal Paediatrics.

Dr Duane Alexander, director of the National Institute of Child Health and Human Development, in Maryland, America, who took part in the research said: "Vitamin B12 is essential for the functioning of the nervous system and for the production of red blood cells.

"The results of this study suggest that women with low levels of B12 not only may risk health problems of their own, but also may increase the chance that their children may be born with a serious birth defect."

Blood taken from one group of 160 women who were pregnant with a child that had a neural tube defect at the time the sample was taken was compared to women who had previously had a child with a neural tube defect but whose current pregnancy was unaffected.

The researchers adjusted for folate levels in order to evaluate the effect of B12 levels independently of folic acid.

It is not known how deficiency in B12 and folate increases the risk of neural tube defects but the vitamins are involved in several biochemical reactions in the body.

Dr James Mills, senior investigator in the Division of Epidemiology, Statistics, and Prevention Research, in America and co-author said that critical events in the formation of the brain and spinal column occur very early in pregnancy - in the first 28 days after conception - before many women even realise they are pregnant.

"If women wait until they realise that they are pregnant before they start taking folic acid, it is usually too late," he said.

In America all women of childbearing age are recommended to consume 400 micrograms of folic acid each day to ensure they have sufficient levels if they fall pregnant unintentionally.

Dr Mills said it would be wise for women to do the same with B12.

"Our results offer evidence that women who have adequate B12 levels before they become pregnant may further reduce the occurrence of this class of birth defects," Dr. Mills said.