Placenta

Biology

Anatomy or system affected: Circulatory system, endocrine system, reproductive system, uterus

Definition: The organ that develops during pregnancy to connect the mother to the developing fetus through the umbilical cord.

Structure and Functions

The placenta is an important and unique organ that develops in women only during pregnancy. The placenta connects a woman’s body to the embryo and then the fetus through the umbilical cord. The cells that make up the placenta, called trophoblasts, function to control the degree of uterine invasion in the mother and the development of a nutrient, gas, and waste transport for the fetus. The fetus receives oxygen and nutrients from the mother and eliminates wastes through the placenta. The placenta is necessary for the development and survival of the fetus during pregnancy, but it must be delivered from the mother’s body after the baby’s birth. It is then termed the afterbirth.

In addition to its primary goal of transporting nutrients, oxygen, and waste between mother and fetus, the placenta also serves as a major endocrine organ. The placenta synthesizes and secretes sex steroid and protein hormones. One hormone in particular, human chorionic gonadotropin (hCG), is secreted by the placenta about one week after an egg has been fertilized (conception). This hormone stimulates the production of the steroid hormone progesterone, which is needed for survival of the baby. The detection of hCG in a mother’s urine is the most common test for pregnancy.

Disorders and Diseases

In most pregnant women, the placenta forms and grows normally. In some cases, however, the placenta does not grow properly, is poorly positioned in the uterus, or does not function properly. It may be too large or small or connect abnormally. Placental problems are among the most common complications with pregnancy.

Placenta previa is a condition that occurs during pregnancy when the placenta implants in the lower part of the uterus and obstructs the cervical opening to the birth canal. The incidence of placenta previa is approximately one in two hundred births.

Placental abruption during pregnancy is a condition in which the placenta separates from the uterus before the fetus is born. This condition occurs in about one in every ninety deliveries. A woman is more likely to develop this condition if she has preeclampsia. The cause is not known, but preeclampsia usually occurs in the second half of pregnancy. Signs include high blood pressure, swelling, and protein in the urine. The risk of preeclampsia is higher in women carrying multiple fetuses, in teenage mothers, and in women older than age forty. Most women with preeclampsia still deliver healthy babies, but a rare few may develop a condition called eclampsia (seizures caused by toxemia), which is very serious for the mother and the baby. Approximately 8 percent of pregnant women will develop preeclampsia.

When the placenta fails to develop or function properly, the fetus cannot grow and develop normally; this is called placental insufficiency. The earlier in pregnancy that this occurs, the more severe the resulting problems. If placental insufficiency occurs for a long time during pregnancy, then it may lead to intrauterine growth retardation or restriction (IUGR), a condition in which the fetus does not grow as large as it should while in the uterus. These babies are very small for their gestational age. IUGR can be caused by decreased blood flow to the placenta, drug use, smoking, alcoholism, or placental abnormalities. A diagnosis can be made through ultrasound to measure fetal growth and a non-stress test that measures the heart rate and movement of the fetus. Between 3 and 5 percent of all pregnancies are complicated by IUGR caused by placental insufficiency. A baby with severe IUGR is more likely to have health problems in the newborn period and throughout childhood.

Perspective and Prospects

The placenta is a unique organ essential for the birth of a child. The most common problems in pregnancy involve the placenta; fortunately, many are manageable. With preeclampsia, delivery of the baby is the best way to protect both the mother and the baby. This is not always possible, however, because it may be too early for the baby to live outside the womb. In this case, steps can be taken to manage the preeclampsia until the baby can be delivered, including decreasing blood pressure with bed rest or medicine. Fortunately, preeclampsia is usually detected early in women who obtain regular prenatal care, and most problems can be prevented. Most cases of IUGR cannot be prevented, especially if they are the result of genetic causes. Some cases can be prevented by taking the following precautions: abstinence from alcohol, tobacco, and illicit drugs; careful monitoring and early treatment for high blood pressure and diabetes; and a diet high in folate before and during pregnancy to protect against certain birth defects.

Bibliography

Benirschke, Kurt, Graham Burton, and Rebecca Baergen. Pathology of the Human Placenta. 7th ed., Springer, 2022.

Berven, Eirik, and Andras Freberg. Human Placenta: Structure and Development, Circulation, and Functions. Nova Biomedical Books, 2010.

Harding, Richard, and Alan D. Bocking, eds. Fetal Growth and Development. Cambridge University Press, 2001.

Kay, Helen H., D. Michael Nelson, and Yuping Wang. The Placenta: From Development to Disease. Wiley-Blackwell, 2011.

"Placenta." Cleveland Clinic, 4 Feb. 2022, my.clevelandclinic.org/health/body/22337-placenta. Accessed 15 July 2023.

Wynn, Ralph M., and William P. Jollie, eds. Biology of the Uterus. 2nd ed., Plenum, 1989.