Before pregnancy, practice eating a balanced diet and take an over-the-counter multivitamin that contains 400 micrograms of folate, such as Centrum.  For best tolerance, take vitamins with food or before going to bed.  There is no one best prenatal vitamin, although Stuart Prenatal is well-established prescription brand.  Some prenatal vitamins contain stool softeners to treat constipation and others contain higher doses of iron for women prone to anemia.  Several manufacturers have begun to add omega-3 fatty acids to prenatal vitamins, which has pros and cons.  Research prenatal vitamins at About.com and ask your pharmacist for specific recommendations.