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.
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.
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.
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.