placenta

Placenta is the temporary organ that develops during pregnancy to connect the developing fetus to the uterine wall, enabling the exchange of nutrients, gases, and wastes between mother and child. It forms from the fusion of fetal trophoblast tissue with maternal decidua and is attached to the fetus by the umbilical cord. In humans, the placenta is classified as hemochorial, in which maternal blood directly bathes the chorionic villi to maximize exchange surface.

Structure and development: The placenta comprises chorionic villi, which grow into the endometrium, and a maternal component called the decidua basalis. Blood from the mother circulates through intervillous spaces surrounding the villi, while fetal vessels remain within the villi. The placental barrier permits diffusion of oxygen, carbon dioxide, nutrients, and small molecules; larger proteins and cells cross only with restricted efficiency.

Functions and hormones: The placenta serves as the site of nutrient and gas exchange, waste removal, and immunological interaction. It also produces key hormones, including human chorionic gonadotropin (hCG), progesterone, estrogens, and placental lactogen, which help sustain pregnancy, support fetal growth, and modulate maternal physiology.

Clinical and developmental notes: Placental development begins after implantation and is typically fully formed by the end of the first trimester, continuing to mature throughout pregnancy. Abnormal placentation or placental pathologies, such as placenta previa or placental insufficiency, can affect fetal growth and pregnancy outcomes.