Prenatal vitamins help prevent birth defects. One of the most important vitamins is folate (folic acid) which prevents neural tube defects. Folate is a water soluble B-vitamin otherwise known as vitamin B-9. Women should be taking at least 0.4 mg of folate at least 1 month prior to becoming pregnant if there is no personal or family history of neural tube defects. If there is a history of neural tube defects, then it is generally recommended to supplement with folate at least 3 months in advance.
Aside from folate, women should be aware that calcium and iron requirements increase during pregnancy. You should consult with your health care provider to assess your nutritional needs and come up with a plan and vitamin regimen.