OBJECTIVE To determine whether primiparous adolescent pregnancy is associated with increased risk of iron deficiency anemia and iron deficiency when compared with older primiparas. METHODS Prospective cohort. METHODS Caratinga city, Minas Gerais State, Brazil. METHODS A total of 183 women, being 61 adolescents and 122 adults. METHODS Five peripheral vein blood samples were collected from each participant, at 9-16 and 29-36 weeks, upon admission for delivery, within the first hour after delivery and 30-60 days post-partum. We measured hemoglobin (Hb), transferrin saturation index and ferritin. METHODS Adolescent and adult women using chi-squared test, Student's t-test and hotelling test. RESULTS There were significant differences in Hb concentration during the various sampling periods, for the whole cohort. The prevalence of anemia (Hb < 11 g/dl) did not differ significantly between adults and adolescents. The decrease in the mean transferrin levels was more intense in adolescents than in adults, especially during the second half of pregnancy. Low ferritin levels (<12 microg/l) were more frequent in adolescents than in adults during pregnancy and throughout the study period. CONCLUSIONS Although there were no significant differences in the prevalence of iron deficiency anemia (Hb < 11 g/dl) in adolescents compared to adult women, low body iron stores and ferritin (< 12 microg/l) were significantly more frequent in adolescents.