Background: No consensus exists regarding the hemoglobin (Hb) values that define postpartum anemia. Knowledge is currently lacking regarding prescription and consumption practices, which prevents evaluating the rational use of iron supplementation postpartum. Aim: In this study, our objective was to describe this practice and analyze its association with maternal health outcomes. Methods: A prospective observational study was conducted with 1010 women aged between 18 and 50. The hemoglobin value on the first postpartum day; the prescription schedule at hospital discharge; iron consumption; and data on hemoglobin, serum ferritin, maternal fatigue, type of breastfeeding, and perceived health six weeks after delivery were collected. Findings: Oral iron was prescribed to 98.1% of mothers with anemia and 75.8% without anemia. At the same Hb value, the maximum amount of total iron prescribed was between 8 and 10 times greater than the minimum amount. Iron intake was significantly lower than prescribed (p < 0.01). At six weeks, anemic mothers who took iron presented a 3.6-, 3-, and 2.4-times lower probability of iron deficiency, anemia, and abandoning breastfeeding, respectively. Discussion: Postpartum iron intake shows a protective effect on iron deficiency and anemia at six weeks, but not on fatigue or self-perceived health level. Conclusion: We conclude that there is wide variability in the prescription regimen. Oral iron supplementation can benefit mothers with anemia and harm those without. Subsequent studies should further explore the Hb figure that better discriminates the need for postpartum iron.