Forty-two healthy young women, less than 16 weeks pregnant, with normal hematologic status, were selected at random to receive either a multivitamin/multimineral with 65 mg of iron or one without iron. During pregnancy and the puerperium, a full hematologic evaluation of the iron, folate and B12 status was performed. Those receiving the multivitamin without iron had significantly lower mean serum ferritin levels (P less than .05). During the study, 9 of these 21 noniron subjects (43%) failed to maintain an acceptable hemoglobin level (greater than 11.0 g/dL) and were medicated with 110 mg ferrous iron daily. Use of the multivitamin/multimineral supplement with iron during pregnancy and for 12 weeks postpartum maintained maternal iron stores, eliminated the need for additional iron medication and prevented the development of iron deficiency anemia. This 36-week longitudinal evaluation demonstrated the need for iron supplementation during pregnancy and for three months postpartum. Measurements of serum ferritin at 16-20 weeks of pregnancy will detect hematologic risk in young pregnant women not identified by usual hematologic assessments. The results of this study show that adequate maternal iron stores during pregnancy and the puerperium cannot be maintained by prevailing dietary patterns and therefore that iron supplementation is essential.