Development, Function, and Pathology of the Placenta

• The placenta is the first organ to form in mammals and is required for establishment of a maternal–fetal vascular interface capable of supplying the bioenergetic needs of the developing conceptus. • Multiple placental cell types engage in highly varied functions, from attachment, invasion, and vascular remodeling to cell fusion, hormone production, and nutrient transport. • Multiple mechanisms allow transport of waste and nutrients across the placenta, including diffusion, transporter protein-mediated (facilitated diffusion and active transporters), and receptor-mediated mechanisms. • The placenta is not an inert transport interface. It consumes 40%–60% of the oxygen and glucose delivered to the uterus at term. Thus conditions that alter placental metabolism can indirectly affect nutrient transport to the fetus. • Maternal and fetal health alter placental function, which in turn influences fetal adaptations and contributes to in utero fetal programming. • Abnormal placentation and placental infections can lead to preeclampsia (PE), fetal growth retardation, or preterm birth, which can have lifelong bearing on health. • In the United States, iatrogenic delivery is responsible for almost half the births that occur between 28 and 37 weeks of gestation, primarily caused by placental pathologies such as PE or intrauterine growth restriction. • Efforts to standardize placental examination after delivery are in progress so that connections between specific placental problems and poor outcomes can be better defined. In parallel, new advanced imaging techniques and biomarkers for placental function are being developed.