Spina bifida and other neural tube defects (NTDs) occur between the third and fourth week of fetal development, a time when most women don't even know that they are pregnant. Because up to 50% of all pregnancies are unplanned, all women of childbearing age should take a multivitamin containing folic acid every day to help prevent NTDs. Be sure to take a multivitamin that also contains vitamin B12. Folic acid helps reduce the risk of birth defects by more than 70% if taken before pregnancy.