Babies whose mothers had low levels of vitamin B12 just before and after they were conceived could be up to five times more likely to be being born with a congenital defect, a study said Monday.

Those women, who eat little or no meat or animal-based foods, were the most likely to have low levels of vitamin B12 and give birth to babies with a neural tube defect, according to the joint study by Trinity College, Dublin, the Health Board of Ireland and the US National Institutes of Health.

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

"Vitamin B12 is essential for the functioning of the nervous system and for the production of red blood cells," said Duane Alexander, director of the National Institute of Child Health and Human Development.

"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."

It has long been known that mothers-to-be should increase their levels of folic acid, another B vitamin, to reduce birth defects in their children.

The study, published Monday in the March issue of Paediatrics, said such neural tube defects affect the brain and spinal cord, such as spina bifida, which can cause partial paralysis. Another type, anencephaly, is a fatal defect in which the brain and skull are severely underdeveloped.

The researchers analysed stored blood samples originally collected during early pregnancy from three groups of Irish women between 1983 and 1990, when it was not so common for pregnant women to take vitamin supplements, and compared them against the records of whose babies were born with congenital defects.

The study showed that pregnant women with less than 250 nanograms of vitamin B12 per litre of blood had a risk 2.5 to three times higher of having a baby with a neural tube defect.

Women with the lowest levels of vitamin B12 (from zero to 149 nanograms a litre) ran the greatest risk, some five times higher.

Ireland has a high rate of neural tube defects, and NIH scientists have frequently collaborated with Irish researchers to gain insight into the causes of this group of disorders, the authors said in a statement.

The results could mean that women and expectant mothers who stick to a vegan diet may increase the risks of their baby developing some kind of congenital defect.

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

Thus he said all women of child bearing age should also consume the recommended amount of vitamin B12, found in meat, chicken, milk and eggs, whether they are planning a pregnancy or not.

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, Mills said.