Women who don't have enough vitamin B12 in their blood before they conceive have a higher chance of delivering a baby with brain or spinal cord defects, a new study finds.

Most at risk may be vegans and vegetarians, since most meat and animal-based foods already contain the vitamin.

Also at risk are women who have disorders such as inflammatory bowel disease that prevent them from absorbing sufficient amounts of B12.

According to the study, published in the March issue of Pediatrics, women with low levels of B12 have at least 2.5 times the risk of giving birth to a child with neural tube defects, compared to women with adequate B12.

Women with the lowest B12 levels have five times the risk compared to those with the highest B12 levels.

Neural tube defects can lead to lifelong disability. The defects can cause spina bifida, in which the spinal cord and back bones do not form properly, or anencephaly, a condition in which the brain and skull bones do not develop normally and which almost always leads to death.

Women have long been advised to take folic acid supplements to ensure they have adequate folate levels, another B vitamin, in their blood. Since folic acid public awareness campaigns began along with cereal grain fortification, there has been a significant drop in neural tube defects.

But this new research from Ireland finds that that vitamin B12 deficiency is also a risk factor for neural tube defects -- regardless of folate levels.

The researchers suggest that awareness of the similar role of vitamin B12 might be able to reduce neural tube defects further.

The study involved almost 1,200 women in Ireland who gave blood samples during early pregnancy. The women were classified into three groups.

• The first group consisted of 95 women who were pregnant with a child having a neural tube defect at the time the blood was taken.
• The second group was composed of 107 women who had previously given birth to a child with a neural tube defect but whose current pregnancy was not affected.
• Like the first group, women in the third group (a total of 76) were pregnant with a child having a neural tube defect at the time the blood sample was obtained, but were enrolled in a different study than the women in group 1.

In all three groups, women with low B12 concentrations (estimated at less than 250 ng/L, before pregnancy) had 2.5 to 3.0 times the risk of having a child with a neural tube defect compared to those with higher levels. Women with the lowest levels of B12 were at five times the risk of women with the highest levels.

Dr. James Mills of the U.S. National Institutes of Health, one of the researchers, explained 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 realize they are pregnant.

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

Similarly, he said, it would be wise for all women of childbearing age to consume the recommended amount of vitamin B12, whether they are planning a pregnancy or not.

"Half of the women who become pregnant each year in the U.S. were not planning to become pregnant."

Vitamin B12 is available in milk, meats, poultry, eggs, as well as fortified cereals and some other fortified foods.