The role of exercise in pregnancy (EIP) is widely debated. Pregnancy produces marked changes of several physiologic variables of the mother. Regular EIP limits the increase in O2requirement and produces an increase in the absolute anaerobic threshold, which persists for up to 36 weeks. Exercise in pregnancy reduces the incidence of muscle cramps, lower limb edema, fatigue, and shortness of breath. Increased knee cruciate ligament laxity does not normally produce any instability. Exercise in pregnancy reduces the increase in baseline maternal heart rate that occurs in pregnancy. Heart rate and stroke volume increase more markedly with exercise in pregnant women than in nonpregnant controls. Noradrenalin response to EIP is lower than in nonpregnant controls. Blood glucose levels decrease more promptly and to lower values with exercise in pregnant women than in nonpregnant controls. Glucose tolerance normally decreases in pregnancy, but increases during EIP. Regular EIP improves glucose tolerance in diabetic mothers and reduces total maternal weight gain and subcutaneous fat deposition toward the lower end of the normal range. Regular EIP produces higher plasma endorphin levels during labor and better pain tolerance than in sedentary controls. Women who undertake regular EIP have a lower incidence of 3rd- and 4th-degree vaginal tears. Children of exercising mothers have similar birth-weights as children of sedentary mothers, and mental performance at age 5 is higher. If there are no specific obstetric or medical contraindications, fit pregnant women can safely maintain the same level of fitness during pregnancy, although exercise schedules may have to be reduced.