Since many physiological, metabolic, and endocrine changes that occur during pregnancy are evident even at rest, the alterations found during exercise may not necessarily be the same as those found in the normal population. Nonetheless, the exercise-induced cardiopulmonary changes are essentially normal or slightly exaggerated during pregnancy. The energy cost of cycle exercise is unchanged during pregnancy; however, the increased weight bearing, especially evident in late pregnancy, adds to the exercise effort during walking, climbing, or jogging. Aerobic work capacity remains unchanged during pregnancy, and typical training adaptations can be found during pregnancy. Hypoglycemia occurs more easily during exercise in pregnant women, even though lipid provision is exaggerated during late pregnancy. The influence of maternal exercise on the fetus is evident in changed heart rhythm and breathing patterns of the fetus. Pregnant patients with utero-placental insufficiency are more likely to have these fetal changes during exercise. Severe hyperthermia should be avoided during pregnancy. Animal studies indicate that some aspects of fetal metabolism are affected by maternal exercise; whether the reduction in uterine blood flow found during heavy exercise exacerbates this response is not known. Birth weight is unaffected when healthy well-nourished mothers participate in mild to moderate exercise programs during pregnancy. However, more intense exercise programs during pregnancy in animals can cause changes in fetal growth and litter size.