Folic acid is a B vitamin. When taken before and early in pregnancy it helps prevent birth defects of nervous tissue, the brain and spinal cord. Some studies show it can also prevent miscarriage, stroke, autism, and certain cancers. Every fertile woman should take a multivitamin with 400 micrograms of folic acid before pregnancy as well as during early pregnancy. Women who have already had a pregnancy affected by a birth defect of the brain or spinal cord, or a child with autism spectrum disorder (ASD) should take 4000 micrograms.