Placenta

Structure and main functions

Structurally the placenta has maternal and fetal surfaces. The fetal side is formed by chorionic villi and is connected to the fetus by the umbilical cord, which typically contains two fetal arteries and one fetal vein. The maternal side develops from endometrial tissue. Together these components create surfaces and small spaces where exchange of gases, nutrients and wastes occurs without direct mixing of maternal and fetal blood in most species.

• Gas exchange: oxygen moves from mother to fetus; carbon dioxide moves from fetus to mother.
• Nutrient transfer: glucose, amino acids, lipids and other substrates support fetal growth.
• Waste removal: fetal metabolic byproducts are transported to maternal circulation for clearance.
• Endocrine function: the placenta secretes hormones such as human chorionic gonadotropin, progesterone precursors and estrogens that support pregnancy.
• Immune and barrier roles: it contributes to immune tolerance and provides selective protection against pathogens and maternal antibodies.

Development and lifecycle

Placental development begins at implantation when trophoblast cells from the early embryo invade the uterine lining and differentiate into specialized tissues. Over the course of the first trimester the placenta expands and becomes progressively more efficient at exchange. The organ remains active throughout pregnancy and undergoes structural and hormonal changes as the fetus grows. After the baby is born, uterine contractions expel the placenta and cord; this expelled tissue is commonly called the afterbirth. Medical attention to the timing and completeness of placental separation is important to prevent bleeding and infection.

Clinical significance and human interactions

Because the placenta is essential for fetal health, abnormalities in its location, attachment or function can affect pregnancy outcomes. Examples include placenta previa (low-lying placenta), placental abruption (premature separation), and placental insufficiency (reduced exchange leading to growth restriction). The placenta is also a target for prenatal diagnostic procedures such as chorionic villus sampling, and its condition is examined after delivery in many clinical settings.

Beyond clinical care, the placenta has cultural and historical roles in some societies. Practices such as placenta burial or ceremonial handling reflect cultural meanings attached to birth. Some people choose placentophagy (consumption of placenta) after delivery; this practice is controversial and scientific evidence of medical benefit is minimal and uncertain.

Variation among mammals and notable facts

Placental structure varies widely across mammal species. In humans the placenta is described as discoid and hemochorial, meaning a disc-shaped organ in which maternal blood has relatively direct contact with the chorionic villi. Other mammals have zonary, cotyledonary or diffuse placentas and different tissue arrangements that affect the nature of maternal–fetal exchange. The placenta also plays a key role in evolutionary biology and immunology because of its involvement in maternal–fetal conflict, immune tolerance and the coevolution of reproductive strategies.

For further general information see a basic medical source on the topic (temporary organ) or materials that describe placentation in different species (pregnant female mammals). Practical details about the umbilical connection are often shown in patient information about the umbilical cord.