It depends on the woman's health and diet. Some women are predisposed to have low iron in their blood, but they also do not eat properly to balance that. Thus, for the sake of the baby, a supplement might be required; however, one should be very careful with supplements since most are chemically synthesized and are filled with impurities. It is always best to acquire our nutrition from nature: if a woman is found to be needing vitamins or minerals, she will do better to find natural sources for them (such as eating more of the appropriate foods).