In 1559, a man named Realdus Columbus called it placenta after the Latin word for "circular cake." (Ref., Williams Obstetrics, 18th edition.) It makes us all possible.
The placenta is part of the communication between the fetus and the expectant mother. Most people tend to think of this communication as the route of exchange where the mother's blood and the fetus's blood mix and exchange, but this is a myth. The fetal blood and maternal blood do not mix. In fact, if this were to be the case, there would be such immunological protest from the mother that she would soon make enough antibodies to the baby's blood to destroy the pregnancy. So what exactly is meant by communication between the fetus and mother? And if no blood is exchanged, how do oxygen and nutrition get passed on to the developing baby?
Before I address this question, there is another related misconception. For the longest time, it was perceived that the baby was just along for the ride, not actively doing anything. "It is now known that this is not the case. To comprehend the nature and importance of the fetal-maternal communication system of human pregnancy, it is important to acknowledge that the fetus enjoys a position of protection from the external environment that is never to be experienced again in life. At the same time, it must be recognized that the fetus is the dynamic force in the orchestration of its own destiny." (Williams)
The answer to how both those factors come into play (maternal/fetal communication and how the fetus orchestrates its destiny) lies with the blastocyst, an early cellular developmental stage of the soon-to-be embryo, which is the principal influence in its own implantation (nidation) in the uterus (womb). From this early on there are chemical communications, and these chemical and hormonal messages continue in the best interest of the pregnancy until birth. Labor and lactation (milk let-down) are dependent on this communication.
The blastocyst causes the irritation that alters the lining of the uterus in such a way as to allow implantation. The pregnancy hormone, hCG, is made by the blastocyst and embryo. This hormone causes that part of the ovary that makes progesterone (necessary for the rich lining of the womb), to continue doing so until the fetal tissue can make its own. There are chemicals called prostaglandins that are normally made by the lining when pregnancy does not take place. These chemicals, in turn, cause that part of the ovary that was previously making progesterone to quit. The implanting blastocyst then prevents this production of prostaglandins, the ovary continues making progesterone, and the implanted embryo is therefore protected by the rich uterine lining.
Prostaglandins, by the way, are just one ingredient in the gumbo of biochemistry that initiates labor nine months later; so it appears that in keeping with the phrase "that the fetus is the dynamic force in the orchestration of its own destiny," the fetus also contributes signals that promote the onset of labor and when to be born.
The mother's immune system doesn't reject the baby as a foreign transplanted organ because the blastocyst suppresses the irritants that promote rejection (called HLA antigens). Williams called this, "Fetal contributions to the maternal acceptance of the...fetal graft." Because of this, "the placenta and...fetal membranes appear to defy the laws of transplantation immunology."
Try transplanting a man's kidney--or even a rib!--into his wife, and there will be trouble for sure. Progesterone, made from cholesterol (of all things!) and made in abundance (luckily for us), seems to make a wonderful anti-inflammatory agent. We know this isn't the only chemical allowing the maternal tissue to forgive this invasion, but we also know that it plays an important role.
Another thing that helps is the complete separation of the maternal and fetal circulation. As stated above, there is no direct connection between the fetal bloodstream and the maternal bloodstream. The placenta grows into the maternal uterine lining, but no blood vessels connect up to the mother's, as was once thought. Instead, the fetal red blood cells end their journey in a U-turn in very vascular capillaries; these capillaries sit in puddles, so to speak, called intervillous spaces. These are puddles of maternal blood that bathe the lining of the placental capillaries. The fetal tissue, thus bathed, takes the developing baby's needs out of the maternal blood, leaving the rest for the mother."
Special tissues of the placenta, cells called "trophoblasts," are the cells that receive diffused and transferred substances from the mother's blood (those so-called puddles). This interchange, which includes oxygen, carbon dioxide, nutrients going in, and waste going out, works to effectively provide the lungs, kidneys, stomach, and intestines with their needs.
Labor has been designated as having official stages. The first two stages involve labor and delivery of the infant, and the third stage is when the placenta is delivered. It must be remembered that the uterus is a big sphere of muscle with an opening in the right place to expel a baby. When that baby no longer occupies the space inside the uterus, the uterus contracts down. As with a balloon that's been untied, the amount of surface area within gets smaller.
The placenta, once separated, follows the way of its companion—the baby—to the outside world.