This question has been the source of much debate. The medical community is not totally convinced that prenatal vitamins alone are the source of increased hair growth during pregnancy. This is because not all pregnant women who take prenatal vitamins experience this positive result with their hair.
But, we do know that prenatal vitamins contain large amounts of nutrients including iron, zinc and folic acid which provide nourishment to the cells in the scalp. So it is possible that many pregnant women experience hair growth because during this time they are taking in a sufficient amount of these nutrients.
If your typical daily diet consists of unprocessed foods, fruits, a colorful variety of vegetables, whole grains, lentils, and plenty of water, then you will likely have sufficient vitamins and minerals needed for growth and health of your hair.