Doctors generally advise pregnant women to get folic acid through their diet or vitamin supplements. Taking folic acid, or vitamin B9, before or during early pregnancy is known to protect babies against birth defects.

Findings of a new study, however, suggest that women who have too much folate and vitamin B12 may increase their baby's risk of developing autism spectrum disorder, a condition marked by difficulties in verbal and nonverbal communication and social interaction problems.

For the research, Daniele Fallin, from the Johns Hopkins Bloomberg School of Public Health, and colleagues looked at the data of nearly 1,400 mother-child pairs.

Ten percent of the women in the study were found to have excess amount of folate, and 6 percent had excess amount of vitamin B12.

Fallin and colleagues found that women with very high levels of folate during delivery had twice the risk to deliver a child with autism than women with normal levels of folate in the blood.

They also found that women with excessive B12 levels had three times increased odds of having a child with autism.

The risk though was highest among women whose blood contained excess amounts of both B12 and folate. The risk for women in this group to have a child with autism is more than 17 times compared with women with normal levels of both nutrients.

Despite their findings, the researchers said that their study does not prove a causal relationship, which means it only found a link and did not provide evidence that high levels of the nutrients may increase risk of autism.

The researchers also acknowledged they could not determine why some women had excessive amount of folate or B12 at the time of birth.

Fallin said it is possible that some women are just genetically predisposed to have high levels of B12 and folate, or they may be getting too much of these nutrients from their diet or supplements. Folate is naturally found in fruits and vegetables. Folic acid is also used to fortify cereals and breads in the U.S.

The researchers added that their findings do not suggest that women should throw away their supplements.

"We have long known that a folate deficiency in pregnant mothers is detrimental to her child's development. But what this tells us is that excessive amounts may also cause harm. We must aim for optimal levels of this important nutrient," said Fallin.

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