BienFer is a double-blinded randomized controlled trial about iron substitution and abdominal pain. Premature babies and low birth weight babies have lower iron stores than term newborns and are therefore at greater risk of iron deficiency and anemia. Iron deficiency could have consequences for cerebral development. Ferric substitution reduces the risk of iron deficiency and anemia. It is common to replace premature babies with iron substitution between 2 weeks and 6 months of life. In practice, the medical staff notice that newborn with iron substitution have abdominal pain and constipation. A study in the adult population shows that alternate iron supplementation versus daily supplementation improved iron absorption, with no difference in hemoglobin level and decrease abdominal pain. Studies in children shows that intermittent iron supplementation do not change hemoglobin and ferritin level and improve adherence. There is no data comparing daily versus alternate supplementation in premature babies. The aim of the study is to evaluate if alternate (4 times/week) iron supplementation is equivalent to daily iron substitution for hemoglobin values in premature babies between 28 0/7 weeks amenorrhea (WA) and 34 6/7 WA, and if it reduces abdominal discomfort and inflammation, and if it improves compliance.