From Encyclopedia of Sex and Sexuality
The placenta is the envelope within which a fetus grows and develops in the mother’s uterus (womb) during the nine months of a normal pregnancy. It is in the placenta that the blood supply of the future baby and the mother come into intimate contact, allowing the transfer between mother and fetus of the nutritional and other chemical substances essential to the fetus’s growth and development. Thus, the placenta serves as the organ of respiration, nutrition, and excretion for the future baby while it is developing within the mother.
The placenta is formed early in the pregnancy when finger-like projections called trophoblasts enter the lining of the mother’s uterus. Blood from the mother reaches the placenta through blood vessels in the uterus, and blood from the fetus reaches the placenta through two umbilical arteries located in the umbilical cord, which extends from the fetus’s navel to the placenta.
The mother’s blood never circulates inside the fetus, nor does the fetus’s blood circulate inside the mother. The nutrition and substances needed by the fetus can cross the “placental barrier,” while many organisms cannot. Thus, the mother may be suffering from a bad cold and the fetus will remain healthy. However, many viruses and disease organisms can pass through the placental barrier, among them the AIDS virus, syphilis, and German measles. Some drugs taken by the mother can also pass through the barrier.
The placenta itself produces many of the hormones necessary for proper regulation of the pregnancy. For example, it makes human chorionic gonadotropin (hCG), which is the hormone measured by pregnancy tests. Hormones from the placenta help alter the mother’s body to accept and adjust to pregnancy and to help her develop the ability to nurse the newly born child. The placenta makes several steroid hormones as well and is capable of producing progesterone from substances obtained from the mother. The progesterone in turn helps to maintain the developing pregnancy. Using hormones produced by the fetus, the placenta can also produce high levels of estriol, the estrogen most plentiful during pregnancy.
During the pregnancy, all the nutrients required for sustenance and development of the future baby are transferred to the fetus through the placenta from the mother’s blood supply. The oxygen needed by the fetus to live is transferred through the placenta to the fetal blood stream from the mother’s blood supply. Similarly, the waste products from the fetus are excreted by the mother after they have been transferred to her through the placenta.
A healthy placenta is critical to survival of the future baby. If anything happened to separate the placenta from the mother’s uterus, such as an abdominal injury from an automobile accident, the fetus would immediately suffocate and die. After the new baby is delivered, the placenta usually separates from the mother’s uterus spontaneously and is also delivered (it is then called the “afterbirth”; see also Birth).
Usually the placenta is discarded with other biological wastes after birth, but under special circumstances the placenta and its products may be salvaged after birth of the baby. The hCG hormone can be extracted from the placenta, and steroids and protein hormones can be and have been extracted from it for experimental use. Placenta collection is limited to a fairly small number of hospitals. Obviously, the mother has a say in what happens to the placenta, although the issue is rarely raised.
Interestingly, humans are the only known mammals that do not eat the placenta after birth of their young. Other primates do eat it after delivery. In some primitive human cultures the placenta is given a ceremonial burial (see also High Risk Pregnancy).