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Table of Contents

Magill’s Medical Guide, 9th Edition

Placenta

by Thomas L. Brown, , PhD

Category: Biology

Also known as: Chronic adrenal insufficiency, hypoadrenocorticism

Anatomy or system affected: Fetus, uterus

Specialties and related fields: Obstetrics and gynecology, perinatology

Definition: An organ that attaches to the uterus wall, provides oxygen and nutrients to the baby, and removes waste products from their blood.

Key terms:

afterbirth: the placenta and fetal membranes discharged from the uterus after birth

placenta abruption: when the placenta separates early from the uterus

placenta previa: a condition in which the placenta partially or wholly blocks the neck of the uterus

trophoblasts: cells forming the outer layer of a blastocyst, which provide nutrients to the embryo and develop into a large part of the placenta

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. Still, it must be delivered from the mother’s body after the baby’s birth. It is then termed 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 steroids and protein hormones. One hormone, 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 the baby’s survival. The detection of hCG in a mother’s urine is the most common pregnancy test.

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 of 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.

Placenta abruptio during pregnancy occurs when the placenta separates from the uterus before the fetus is born. This condition occurs in about one of every ninety deliveries. A woman is more likely to develop this condition if she has preeclampsia. The cause is unknown, but preeclampsia usually occurs in the second half of pregnancy. Signs include high blood pressure, swelling, and protein in the urine. Preeclampsia risk is higher in women carrying multiple fetuses, teenage mothers, and women older than forty.

Most women with preeclampsia still deliver healthy babies. Still, a rare few may develop a condition called eclampsia (seizures caused by toxemia), which is very serious for the mother and the baby. Approximately eight percent of pregnant women will develop preeclampsia.

When the placenta fails to develop or function properly, the fetus cannot grow and develop normally; placental insufficiency—the earlier in pregnancy, the more severe the resulting problems. Suppose placental insufficiency occurs for a long time during pregnancy. In that case, 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. However, delivery under such conditions is not always possible because it may be too early for the baby to live outside the womb. In this case, the obstetrician and her team can take steps 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 mothers can prevent IUGR cases 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 congenital disabilities.

For Further Information:

1 

de Bellefonds, Colleen. “What Is the Placenta? What This Organ Does and How It Forms.” What to Expect, 9 Jul. 2021, www.whattoexpect.com/pregnancy/placenta. Accessed 10 Apr. 2022.

2 

Placenta: How it works, what’s normal. Mayo Clinic, 25 Mar. 2020, www.mayoclinic.org/healthy-lifestyle/pregnancy-week-by-week/in-depth/placenta/art-20044425. Accessed 10 Apr. 2022.

3 

Placenta Previa. March of Dimes, Jan. 2022, www.marchofdimes.org/complications/placenta-previa.aspx#. Accessed 10 Apr. 2022.

4 

Placental Insufficiency. MedlinePlus, 1 Apr. 2022, medlineplus.gov/ency/article/001485.htm. Accessed 10 Apr. 2022.

5 

“Understanding the Placenta.” YouTube, uploaded by Zero to Finals, 5 Jul. 2018, www.youtube.com/watch?v=gbextFwqnY4.

6 

What is the Placenta? Pregnancy, birth & baby, Aug. 2020, www.pregnancybirthbaby.org.au/what-is-the-placenta. Accessed 10 Aug. 2022.

7 

What is the Placenta? You and Your Hormones, Feb. 2018, www.yourhormones.info/glands/placenta/. Accessed 10 Apr. 2022.

Citation Types

Type
Format
MLA 9th
Brown, Thomas L. "Placenta." Magill’s Medical Guide, 9th Edition, edited by Anubhav Agarwal,, Salem Press, 2022. Salem Online, online.salempress.com/articleDetails.do?articleName=MMG2022_1072.
APA 7th
Brown, T. L. (2022). Placenta. In A. Agarwal, (Ed.), Magill’s Medical Guide, 9th Edition. Salem Press. online.salempress.com.
CMOS 17th
Brown, Thomas L. "Placenta." Edited by Anubhav Agarwal,. Magill’s Medical Guide, 9th Edition. Hackensack: Salem Press, 2022. Accessed September 16, 2025. online.salempress.com.