The best time to start taking prenatal vitamins is before you become pregnant.
Ideally you would begin taking prenatal vitamins with adequate folic acid, iron and calcium beginning at least three months before you began to try to conceive.
Adequate amounts of folic acid supplement are especially important because they help prevent neural tube birth defects which can occur in your baby so early after conception that you may not even know you are pregnant yet.
There are essentially two types of prenatal vitamins: over-the-counter, and prescription. Over-the-counter prenatal vitamins generally contain everything a mother-to-be needs to supplement her diet for a healthy pregnancy.
Prescription prenatal vitamins may contain extra iron and stool softeners (which offset the extra iron and the natural tendency toward constipation pregnant women have).
If your prenatal vitamins causes you to feel nauseated, try taking it with your largest meal of the day, or at bedtime. If your prenatal vitamin causes you diarrhea, constipation, or intestinal discomfort, you may need a prenatal vitamin with lower iron content.
You should never take more than one prenatal vitamin a day, nor should you take any other vitamin supplements while you are pregnant or trying to conceive without first discussing it with your health care provider.
Always remember that prenatal vitamins are not a substitute for eating a healthy diet. They exist only as an extra bit of insurance that you and your baby are getting the vitamins and minerals you need.