Dr. Ellen Wilson of the University of Texas Southwestern Medical Center in Dallas said about 3,000 U.S. babies annually are born with neural tube defects such as spina bifida, marked by the incomplete closing of the spine and skull.
These birth defects typically happen in the first few weeks of pregnancy before a woman is aware she is expecting -- and since half of all pregnancies in the U.S. are unplanned, it is important for women of child-bearing age to get enough folic acid, Wilson said.
"There's strong evidence that taking the supplement before and during pregnancy reduces the incidence of neural tube defects," Wilson said in a statement. "It may also prevent cleft lip/palate and heart abnormalities."
There has been a decline in neural tube defects since 1998, when grain products were first fortified with folic acid, Wilson said.
Although folic acid is found in breads, pasta and green leafy vegetables, many adult females do not consume enough and may need to take a supplement.
Most over-the-counter prenatal vitamins have approximately 800 micrograms --1 micrograms is 1/1,000th of a milligram -- but higher doses might be obtained by prescription, Wilson said.
"Women planning for pregnancy should take at least 400 mcg daily if they are low risk and 4,000 mcg daily if they have had a child with a neural tube defect or if they take medication for epilepsy," Wilson said.