The placenta is the only temporary organ in the human body. It develops with pregnancy and is shed after the pregnancy ends. It is comprised solely of fetal cells, and then "invades" the mother's uterine wall in an intricate process called placentation.
It is connected to the mother by a network of small blood vessels, and to the fetus through the two arteries and a vein contained within the umbilical cord.
The placenta begins to form the moment the fertilized egg (which has already divided into a clump of cells called a blastocyte by this time) implants in the uterine lining. The placenta continues to grow throughout pregnancy, ultimately becoming roughly disc-shaped, with an average weight of 1 pound at full-term.
The functions of the placenta are:
- To carry oxygen and nutrients from the mother’s circulatory system to the fetus’
- To carry wastes and carbon dioxide from the fetal circulation to the maternal circulation
- To provide “passive immunity” to the fetus by transporting IgG antibodies
- To “filter” microbes to prevent a fetus from getting infectious diseases, though this function is not 100% effective
- To secrete progesterone, human chorionic gonadotropin (hCG), human placenta lactogen (hPL), and estrogen necessary to maintaining pregnancy
- To protect the fetus and the fetal component of the placenta from the mother’s immune system -- which normally attacks "foreign" elements in the body -- by secreting various chemicals that "confuse" and suppress the immune system
- To act as a reservoir of blood for the fetus in case the mother’s circulation is compromised by changes in blood pressure
If any of these functions are impaired, a pregnancy may not be able to continue to full-term. In fact, problems with the placenta are one of the leading causes of pregnancy loss.
Placental problems include:
- Placenta previa -– When the placenta grows over, or close to, the internal opening of the cervix. Previa is associated with a high risk of vaginal bleeding in pregnancy, and can be a life-threatening emergency if a woman begins to labor with a previa.
- Placenta accreta -– If the placenta attaches too deeply into the uterus, it is called an accreta. There are different types of accreta, depending on how deep the attachment occurs. Accreta can also be a life-threatening emergency in labor, and poses a risk for both post-partum hemorrhage and surgical interventions, including hysterectomy.
- Placental abruption -– When a placenta separates from the uterine wall prior to birth, it is called an abruption. It can be fatal for a fetus, depending on the degree of separation. This condition can also be dangerous for the mother, due to excessive blood loss. The only "cure" for a severe abruption is immediate delivery.
- Chorioamnionitis –- A bacterial infection of the membranes that make up the bag of waters. Usually, the infection travels up through the cervix from the vagina. It requires antibiotic treatment and rapid delivery of the fetus to prevent further complications for both mother and baby.
Because problems with the placenta are such a common cause for pregnancy loss, doctors will often recommend that a pathologist examine the placenta after delivery. A placental exam is an essential part of an autopsy of an infant in the case of miscarriage or stillbirth. Your doctor will respect your wishes if you do not want to have an autopsy, but most women and cultures/religions are comfortable with a placental exam, which may result in helpful information about the cause of your loss.
Some cultures have special practices regarding the placenta after birth. Some, like the Maori of New Zealand, the Navajo of North America, and Cambodians, bury the placenta. Among the Ibo in Nigeria, full funeral rites are given to each placenta. Practices worldwide are wildly diverse: exposing the placenta to the elements, planting the placenta along with a tree, even eating the placenta. Placenta is also an ingredient in some Eastern medicines.
In the case of pregnancy loss, if you wish to have your placenta buried or cremated along with your baby, notify your physician.
Oyelese, Yinka, and John C. Smulian, "Placenta Previa, Placenta Accreta, and Vasa Previa." Obstetrics & Gynecology 2006. Accessed 15 Nov 2011.
Varney, H., Kriebs, J., et al. Varney's Midwifery, Fourth Edition. 2003.
What You Need to Know About Your Placenta. March of Dimes. Accessed: 15 Nov 2011. http://www.marchofdimes.com/pnhec/188_1132.asp