You can familiarise yourself with the basic structure and development of the human placenta by reading the appropriate chapters in a textbook of human embryology.
Particular points to consider include:
1. To act as an anchor attaching the embryo to the uterine wall.
This is achieved by the anchoring villi which form the cytotrophoblastic shell.
2. To synthesise hormones.
In the second week of gestation the placenta synthesises human chorionic gonadotropin (hCG), which maintains the corpus luteum, which in turn produces progesterone and oestrogen, which are responsible for maintaining the pregnant state and preventing spontaneous abortion. By 11 weeks the corpus luteum degenerates and the placenta takes over the production and secretion of these hormones. The placenta also secretes a number of other hormones.
3. To act as a conduit allowing the transfer of nutrients from the mother to the embryo/foetus and removal of its waste. In the mature placenta blood from the mother enters the intervillous spaces of the placenta through about 100 spiral arteries, bathes the villi, and leaves again via the endometrial veins. Nutrients and oxygen pass from the maternal blood across the thin villous wall (by both passive and facilitated diffusion) into the foetal blood and waste products such as carbon dioxide and urea pass from the foetal blood to the maternal blood.
The conventional view that the placenta acts as a protective barrier between the embryo and mother is an oversimplification.
Drugs and other chemicals in the maternal blood with a molecular weight of less than 600 daltons (most drugs) can readily cross the placenta and enter the foetal blood. Other larger molecules such as insulin cannot cross the placenta. Some large molecules, such as antibodies (mainly IgG) can cross from the mother to the foetal circulation by receptor-mediated endocytosis and give the newborn limited passive immunity against some infections. Also a few viruses and other infectious agents can cross the placenta and infect the foetus.
The mechanism by which some pathogens cross the thin villous wall is poorly understood. Because the foetus has only a very immature immune system, it is inept at fighting infections; hence a disease that is mild in the mother may damage or kill the foetus. Viruses that can cross the placenta include rubella virus, human parvovirus, varicella-zoster virus and cytomegalovirus.