This chapter reviews the major physiological adaptations during pregnancy and also highlights changes in the reference ranges of common laboratory values encountered in pregnancy. Pregnancy induces a myriad of changes involving the cardiovascular system, respiratory system, gastrointestinal and hepatobiliary systems and genitourinary system. Pregnancy is associated with an overall increase in the serum concentrations of total cortisol, free cortisol, aldosterone, deoxycorticosterone, corticosteroid binding globulin, and adrenocorticotropic hormone. Pituitary enlargement occurs in pregnancy by estrogen mediated proliferation of prolactin-producing cells. During the first trimester of pregnancy, total thyroxine and total tri iodothyronine concentrations begin to increase and peak at mid-gestation, primarily as a result of increased production of thyroid-binding globulin. The immunological adaptations of pregnancy, particularly at the maternal-fetal interface, comprise complex mechanisms that enable the fetus to grow while preventing the mother from rejecting the fetus.