PROJECT SUMMARY / ABSTRACT Postpartum women often are inadequately protected against rapid repeat pregnancy. Ensuring adequate inter- birth intervals could prevent an estimated 9% of deaths worldwide among children less than 5 years of age. Intramuscular injectable depot medroxyprogesterone acetate (DMPA) is rapidly becoming the method of choice in some settings, including regions where high maternal and child mortality make birth spacing critical. DMPA possesses many advantages for postpartum contraception as compared to other methods. However, the World Health Organization (WHO) advises against use of progestin-only injectables during the first six weeks postpartum among breastfeeding women. In contrast, the Centers for Disease Control and Prevention (CDC) recommends that progestin-only injectables generally can be started immediately postpartum on the grounds that their known advantages, as a whole, outweigh their unknown risks. This inconsistency in guidance reflects the lack of high-quality data for making evidence-based decisions. Studies conducted, to date, have important limitations: short follow-up intervals, low power, lack of consistency in using sensitive and standardized assessments, and lack of randomized trials evaluating DMPA administration specifically in the immediate postpartum period. We propose to evaluate the effects of immediate postpartum initiation of DMPA on breastfeeding and on child development. We will conduct a randomized controlled trial of 429 adult women who have delivered a healthy, full-term infant, intend to breastfeed for ≥6 months, and want to use DMPA. Women will be randomized to receive within 48 hours of delivery: 1) DMPA (“intervention” arm), 2) placebo injection (“placebo” arm) or 3) no injection (“open control” arm). The first two arms will be blinded while the open control arm will be unblinded. We will determine the effect of immediate postpartum initiation of DMPA on breastfeeding (Aim 1) and on contraception use (Aim 2). The proposed trial is innovative in use of 1) a randomized, partially-blinded design with sufficient power and follow up; 2) standardized, validated measures on lactation as well as breastfeeding and contraception behaviors; and 3) whole-body air displacement plethysmography to identify differences between arms in infant body composition. The findings of a pilot study in the target population support the feasibility of the proposed trial. We expect the trial findings will permit the harmonization of the WHO and CDC guidance on the timing of DMPA initiation among breastfeeding women. This would have important implications for shaping global policy and practice worldwide, especially in settings where inadequate birth spacing contributes to high maternal and infant morbidity and mortality.