Irish women with the lowest vitamin B12 levels were five times more likely to have a baby with a neural tube defect than those with the highest levels, the researchers wrote in the journal Paediatrics.

Neural tube defects can lead to lifelong disability or death. The two most common ones are spina bifida, in which the spinal cord and back bones do not form properly, and anencephaly, a fatal condition in which the brain and skull bones do not develop normally.

Dr James Mills of the US National Institutes of Health, one of the researchers, said the study showed that vitamin B12 deficiency was a risk factor for neural tube defects independent of folic acid, another B vitamin.

Many women now know of the importance of folic acid and there has been a drop in neural tube defects.

Mills said he hopes that awareness of the similar role of vitamin B12 can reduce neural tube defects further.

Vitamin B12 is essential to maintain healthy nerve cells and red blood cells. It is found in meat, dairy products, eggs, fish, shellfish and fortified breakfast cereals. It also can be taken as an individual supplement or in a multivitamin.

"An absolutely critical point is that women have to consider this before they become pregnant because once they realize they are pregnant it's likely to be too late," Mills, a researcher in the NIH's National Institute of Child Health and Human Development, said in a telephone interview.

The developmental events involved in these birth defects occur in the first four weeks of pregnancy, Mills said.

Mills urged women who do not eat meat or dairy products to be particularly aware of the need to get enough vitamin B12.

He had similar advice for women with an intestinal disorder such as inflammatory bowel disease that may prevent them from absorbing sufficient amounts of the vitamin.

The study involved almost 1,200 women in Ireland who gave blood samples during early pregnancy, which were analyzed to determine vitamin B12 levels.

The women in the lowest 25 per cent of vitamin B12 levels were five times more likely than those in the highest 25 per cent to have had a baby with a neural tube defect.