From The School of Biomedical Sciences Wiki
The placenta is an organ that is joined to the uterus in pregnant women in order to provide a connection between the foetus and the mother's blood stream. This allows movement of nutrients, oxygen, water and IgG antibodies into the foetal blood stream, and removal of waste gases and other waste products.
The placenta also acts as an endocrine gland, secreting hormones essential for the maintenance of pregnancy, such as oestrogen and progesterone. It also must secrete Neurokinin-B to prevent destruction by the immune system of the mother.
The placenta is expelled after giving birth to the baby, and the umbilical cord cut to separate the baby from the placenta