The placenta is a temporary organ that feeds the life growing inside a mother's womb. As the embryo is forming, a yolk sac is created around it. The yolk sac becomes the site of formation for blood that will circulate through the unborn baby. A yolk stalk will form the connection between the embryo and the yolk sac. At the end of the first trimester, the fetus and placenta move apart and stay connected by the umbilical cord.
At week four of pregnancy, chorion villi cover the embryo and a body stock appears. The body stock becomes the connection between the embryo and the chorion villi. The chorion villi look like carpet fibers. These villi extend like many fingers to catch oxygen and nutrients from the mother's blood stream.
As the mother's blood passes over the chorionic villi, fetal blood vessels that are blended in with the villi absorb the oxygen and nutrients. The placenta does not allow the mother's blood to mix with her baby's blood. In this way the placenta acts as a barrier that keeps the baby from being exposed to any disease mother might have.
By the end of the first trimester, the fetus and placenta move apart. They stay connected by the umbilical cord. The yolk stalk is now inside the umbilical cord, along with placental blood vessels. Now blood is flowing from the baby to the placenta and back again. Within the umbilical cord, blood from the placenta flows through a pair of umbilical arteries, and then back through a single umbilical vein. Gases, nutrients, and wastes travel across the chorionic villi.
The volume of maternal blood flow to the placenta and the uterus is heavy, because of the demand that is placed on the blood flow of the mother. Pregnancy is a very delicate time, because the amount of maternal blood flow to the placenta is great. A tear to the placenta could be fatal for both mother and baby.
The placenta also takes the role of an endocrine organ. Hormones from the placenta are released into the maternal bloodstream. These include relaxin that will allow the mother's pelvis to accommodate delivery. Relaxin is the hormone that loosens a woman's ligaments for childbirth. Some experts believe that these hormones, intended for mother, may "spill over" to affect female babies.
These babies are born with congenital hip dysplasia. This would explain why it is girl babies that are born with dislocated hips (like myself, my daughter, and my grandmother) not baby boys.
In addition to releasing hormones, the role of the placenta in fetal circulation is to remove waste and act as a barrier to facilitate the absorption of oxygen and nutrients. Oxygen and nutrients are absorbed from the mother's system, into the system of the unborn child. Chorionic villi allow transfer in a way that keeps the two circulatory systems from mixing. This absorption without mixing is accomplished by the placenta, chorionic villi, and fetal blood vessels that work together in the transfer.