This chapter describes physiological changes that occur in medically uncomplicated pregnancies. Several alterations in immunologic status occur during pregnancy, and among them is a shift away from cell-mediated immunity towards increased humoral immunity. The shift, from a predominance of type 1, or proinflammatory (helper T cells), to type 2, or anti-inflammatory (helper T cells), is considered to have an important role in allowing maternal tolerance of the fetus. An inversion of this shift during the latter half of the third trimester is thought to contribute to the initiation of labor. Every aspect of lipid metabolism is influenced by pregnancy. Maternal serum or plasma cholesterol and triglyceride concentrations increase during pregnancy, with the major increase occurring in the second and third trimesters. In addition to absolute changes in plasma lipid levels, changes in lipoprotein composition also are noted in uncomplicated pregnancy. The apparent physiologic hyperlipidemia of pregnancy is significant for several reasons: (1) it is possible that the increase in plasma triglycerides may enhance the availability of essential and nonessential fatty acids for placental transfer to the fetus, and, as such, is a physiologic adaptation that would serve to favor fetal nutrition and growth; (2) the elevations in cholesterol may provide the excess needed for placental progesterone synthesis and trans-placental cholesterol transfer to the fetus.