placenta /pləsen′tə/ [L, flat cake] , a highly vascular fetal organ that exchanges with the maternal circulation, mainly by diffusion of oxygen, carbon dioxide, and other substances. It begins to form on approximately the eighth day of gestation when the blastocyst touches the wall of the uterus and adheres to it. Placentation begins as the trophoblast is able to digest cells of the endometrium, causing a small erosion on the uterine wall in which an embryo nidates. Human chorionic gonadotropin (HCG), which is chemically identical to luteinizing hormone, is secreted by the developing placenta and promotes survival and hormone production of the corpus luteum. The presence of HCG in the maternal blood and urine is an indicator of early pregnancy. The trophoblastic layer continues to infiltrate the maternal tissues with fingerlike projections, called chorionic villi. By the third month of pregnancy the placenta is able to secrete large amounts of progesterone, enough to relieve the corpus luteum of that function. At term the normal placenta is one seventh to one fifth of the weight of the infant. The maternal surface is lobulated and has a dark red rough, liverlike appearance. The fetal surface is smooth and shiny, covered with the fetal membranes, and marked by large white blood vessels beneath the membranes that fan out from the centrally inserted umbilical cord. The time between the infant’s birth and the expulsion of the placenta is the third and last stage of labor.
