What is the Placenta?

Tara Barnett

The placenta is an organ that develops during pregnancy in many animals, including humans. It is typically born after offspring, and is thus sometimes called an afterbirth. The function of the organ is to connect the mother's body to the fetus, allowing a relatively safe interface between the two bodies. In humans, it is usually disc shaped, reaching around 9 inches (22 centimeters) in length and slightly less than an inch in thickness, and weighing approximately 1 pound (453 grams). In other animals, the size and exact shape of the organ may vary, but the function is much the same.

Smoking can cross the placental barrier, so it is very important not to smoke during pregnancy.
Smoking can cross the placental barrier, so it is very important not to smoke during pregnancy.

One of the most important functions of this temporary organ is passing nutrients and oxygen to the fetus. It is also responsible for getting rid of waste produced by the fetus. It connects to the fetus through the umbilical cord, and to the mother through the uterine wall, where it is firmly anchored. Nutrients, waste, and gases are exchanged through the organ, but blood is never transferred through it.

The placenta transfers oxygen and nutrients to the fetus.
The placenta transfers oxygen and nutrients to the fetus.

This organ is particularly important in some cases because it prevents disorders affecting the mother from affecting the fetus. Blood is never exchanged, and so the fetus has added protection from some problems that are transferred by fluid exchange. Some viruses, as well as toxins from smoking or alcohol, can cross the barrier, and so it is still important for mothers to refrain from dangerous activities and seek help when ill.

A human placenta produces hormones, including progesterone.
A human placenta produces hormones, including progesterone.

In addition to connecting the mother and fetus, a human placenta serves several other functions. It produces hormones, including estrogen, progesterone, and beta human chorionic gonadotrophin. Chemicals secreted by the organ may also help to prevent the mother's body from attacking the fetus and placenta as though it were an invader.

Unfortunately, this important organ is also a source of several problems in pregnancy. Some disorders are caused by the placement of the afterbirth, and may result in bleeding. Others are a result of premature detachment of the organ, which may also cause bleeding. Problems can also be caused by an infection of the afterbirth. These problems are all very serious, and any pain, bleeding, or other unusual symptoms should be promptly investigated to prevent damage to the fetus or mother.

The placenta is a source of many rituals and traditions in human cultures. Some societies bury the placenta, and others eat it, but there are many other ways of ritually disposing of the organ. Many animals are known to eat the afterbirth, which are thought to contain nutrients valuable to the mother. Western medicine has traditionally recommended incineration of the placenta after birth, but some people choose to interact with the placenta rather than dispose it, often electing to eat or donate it for scientific research.

Pain during pregnancy may be a sign of a problem concerning the placenta.
Pain during pregnancy may be a sign of a problem concerning the placenta.

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Discussion Comments


@ MrsWinslow - Exercise does *not* cause placental abruption! Usually, the cause is just a mystery, although sometimes a car accident or that kind of thing can cause it. My cousin experienced an abruption of the placenta and would up spending a couple of weeks on bedrest in the hospital. They gave her steroids to mature her baby's lungs. He was born several weeks early and had to spend some time in the hospital, but they are both home now and doing well.

So placental abruption is definitely serious, but there's not much you can do to prevent it besides generally taking care of yourself. (You aren't smoking or snorting cocaine, I hope.) As for exercise, talk to your doctor or midwife. Most pregnant woman can safely exercise close to their pre-pregnancy level.


My friend told me that too much exercise could cause a placental abruption. Is this true? I've struggled with being overweight and was hoping to continue exercising during my pregnancy. Are there certain kinds of exercise I should stay away from to avoid a placental abruption?

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