Placenta, << pluh SEHN tuh, >> is a disk-shaped organ that develops in pregnant women. The placenta provides the unborn baby with food and oxygen and carries away the baby’s waste products. The organ also produces chemicals called hormones, which maintain the pregnancy and help regulate the baby’s development.
The placenta consists of tissue from both the mother and the embryo. After the first week of pregnancy, the embryo fastens itself to the wall of the uterus, the organ in which the baby develops. The placenta forms as the uterine lining is penetrated and broken down by columns of cells from the chorion, the baglike covering that encloses the embryo. Inside the columns are blood vessels that branch into tiny, fingerlike projections called villi. The villi, which contain the baby’s blood, are surrounded by the mother’s blood. The blood of the baby and of the mother do not mix.
Food and oxygen from the mother’s blood pass through the thin walls of the villi and enter the baby’s blood through a vein in the umbilical cord, a flexible tube connecting the baby to the placenta. Waste products from the baby are carried through arteries in the umbilical cord and pass through the villi. The mother’s circulatory system then gets rid of these wastes.
The placenta expands in size until about 20 weeks. At that time, it covers about half of the internal surface of the uterus. After 20 weeks, the placenta becomes thicker but not wider. At the end of a nine-month pregnancy, it is about 6 to 8 inches (15 to 20 centimeters) in diameter and about 1 to 11/4 inches (2.5 to 3.0 centimeters) thick. It weighs about 14 to 21 ounces (400 to 600 grams).
Minutes after the baby is born, the placenta separates from the uterus. Powerful contractions of the uterus expel the placenta, also called the afterbirth, from the mother’s body.
