The placenta is a transient organ of pregnancy that provides an interface for metabolic exchange between the dam and the fetus.  Placentas are described morphologically according to the distribution of villi on the chorionic surface and the degree of separation between maternal and fetal blood.  The placenta is a transitional endocrine organ that produces hormones responsible for:  1) maintenance of pregnancy; 2) stimulation of the maternal mammary gland and 3) promotion of fetal growth.  Parturition is brought about by production of fetal corticoids and requires removal of the progesterone block.  Parturition consists of three stages.  They are:  1) initiation of myometrial contractions; 2) expulsion of the fetus and 3) expulsion of the fetal membranes.