If you can get pregnant, you should be popping at least one pill a day: a folic acid supplement to lower the risk of a type of serious birth defect in any future offspring.

So says the U.S. Preventive Services Task Force, which Tuesday reiterated its 2009 recommendation that all women who can conceive take 400 to 800 micrograms daily of the B vitamin in case their diet isn't providing enough of it.

Back in 1991, a study found that folic acid supplementation greatly reduced the risk of neural tube defects, a type of birth defect that includes anencephaly and spina bifida. In 1992, the U.S. Public Health Service recommended that women take 400 micrograms as a daily supplement. And starting in 1998, to reach more women, the FDA required that manufacturers fortify cereal and other enriched grains with folic acid, which is the synthetic version of vitamin B9.

Fortification was associated with a drop in neural tube defects, from 10.7 cases per 10,000 live births before fortification to seven cases after fortification, the USPSTF says. (It's not clear how much of that decline was due to fortification.) And not all of these defects are caused by a folate deficiency, so it's not known how that number would change if every woman had enough in the critical early days in pregnancy, often before a woman realizes she is pregnant.

Despite the unknowns, "the evidence is pretty clear that [supplementation] reduces risk ... and it's strong enough that all women who can potentially become pregnant should take it," says Alex Kemper, a pediatrician and member of the Task Force. The recommendation was published in JAMA, the journal of the American Medical Associaition.

For some women, eating fortified foods may do the job, but for others that may not be enough, says Joseph Biggio, who chairs the American College of Obstetricians and Gynecologists' Committee on Genetics. "It's highly diet-dependent," he says. Women on gluten-free or low-carb diets may not be eating fortified grain products. Nor may many be getting enough sources of folate, the naturally occurring version of vitamin B9, including dark, leafy green vegetables, beef liver and beans.

Adolescents as well as ethnic minorities may be at particular risk of deficiency, says Biggio. (Last year the FDA allowed manufacturers to fortify foods made from corn masa, to help prevent birth defects among Hispanic women.) To be safe, ACOG recommends supplementation of 400 micrograms a day.

The neural tube is a part of the embryo that eventually forms the brain and spinal cord. "Picture a pancake that rolls up and forms a tube. That's literally how it forms," says James Mills, a pediatrician and epidemiologist at the National Institute of Child Health and Human Development, who wrote an editorial in JAMA accompanying the recommendations.

That closure happens within 28 days of conception, leaving a short window for women to benefit from supplementation. Not much is known about how the vitamin actually helps prevent defects. So it's unclear, for example, whether a woman who ate a lot of fortified cereal and bread before conception would be OK, or whether she needs to be consistently taking in some form of vitamin B9 throughout those 28 days, says Mills.

Because the critical developmental stage comes so soon after conception, the USPSTF, ACOG and other groups recommend that all women who might become pregnant should take a supplement, not just those who are planning to conceive. Almost half of pregnancies in the U.S. are unplanned, according to the Guttmacher Institute.

Folic acid is available as an individual supplement or as part of a multivitamin. Just check the label to make sure it contains at least 400 micrograms, advises Kemper.

The USPSTF found little evidence of serious harm from folic acid supplementation in women of childbearing age, he says.

One known problem is that it can mask a vitamin B12 deficiency anemia in elderly people, says Mills. For that reason, the Institute of Medicine recommends 1,000 micrograms as an upper limit of folic acid consumption from supplements and fortified foods.

Katherine Hobson is a freelance health and science writer based in Brooklyn, N.Y. She's on Twitter:@katherinehobson.

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