Infants born extremely prematurely are deprived of the placental supply of estradiol (E2) and progesterone (Prog) at an earlier developmental stage compared to an infant born at term. We hypothesized that the retention of Ca (calcium) and P (phosphorus) would be improved by an E2 and Prog replacement. Twenty female infants with a mean gestational age of 26.6 weeks (+/-1.5 SD) and a mean birth weight of 744 g (+/-156) were enrolled in a randomized controlled pilot study. One group received an E2 and Prog replacement to maintain intrauterine plasma concentrations of E2 and Prog and the other group served as control. When intake of formula was at least 100 mL/kg/d, a 3-day Ca and P balance study was performed. Ca and P intake was increased individually until both elements were excreted in the urine. The mean Ca and P retention was 4.21 (+/-1.75) mMol/kg/d (58% of intake) and 2.66 (+/-1.01) mMol/kg/d (80%) in the replaced group and 3.39 (+/-1.69) mMol/kg/d (56%) and 2.03 (+/-0.79) mMol/kg/d (71%) in the control group, respectively. In this pilot study the retention of Ca and P was not improved by an E2 and Prog replacement.