placentas

A placenta is a temporary organ that develops during pregnancy to support the fetus. It mediates nutrient and gas exchange and acts as an endocrine organ to help maintain pregnancy.

In humans and many other mammals, the placenta forms from fetal chorionic tissue and maternal decidual tissue. The fetal portion consists of chorionic villi, covered by trophoblasts, which invade into the maternal blood spaces of the endometrium. The intervillous space is filled with maternal blood, and the umbilical cord contains one vein and two arteries that connect to the fetus.

Exchange across the placental barrier provides oxygen and nutrients to the fetus while removing carbon dioxide and waste. The placenta also synthesizes hormones, including human chorionic gonadotropin (hCG), progesterone, estrogens, and placental lactogen, supporting early corpus luteum function and later sustaining the pregnancy.

Placentation types vary among mammals; the human placenta is hemochorial, meaning maternal blood directly contacts the chorionic villi. Other types include endotheliochorial and epitheliochorial placentas. The placenta participates in immune signaling and remodeling of maternal spiral arteries to increase blood flow.

Clinical relevance: placental dysfunction can contribute to complications such as placenta previa, placental abruption, placenta accreta, fetal growth restriction, and preeclampsia. After birth, the placenta is delivered and is routinely examined for signs of pathology.