To measure infants' breast milk intake and infant growth when their mothers initiated either combined oral contraceptive (COC), levonorgestrel-releasing intrauterine system, or etonogestrel-releasing implant, or copper intrauterine device (IUD) as a reference group. Prospective trial. University-based hospital. On postpartum day 42, 40 women initiated a contraceptive method according to their choice. Deuterium (D2O; 0.5 g/kg mother's weight) was ingested by mothers on postpartum days 42, 52, and 63 as a marker of total body fluid. Infants' milk intake from 42 to 63 postpartum days was assessed by measurement of D2O levels in infants' saliva and infant growth by measuring their body weight, height, and tibia length. Women recorded all infant feed and changes of diapers wet with urine. Breastfeeding continuation was assessed at 6 months postpartum. Infant mean milk intake, mean growth increase, mean number of breastfeeding episodes, daily wet diaper changes, and mean duration of exclusively breastfeeding (~5 months) were similar in the four groups. Use of a COC, the two progestin-only contraceptives, or copper IUD did not affect the amount of infant milk intake and growth up to 9 weeks of age. The incidence of full breastfeeding and breastfeeding continuation was similar with contraceptive hormonal use and no use. NCT01388582.100445-50