The endocrine effects of pregnancy cause striking changes in maternal metabolism, cardiovascular regulation, acid-base balance, and thermoregulation at rest and during standard submaximal exercise. The apparent purpose of these changes is to accommodate fetal needs in addition to those of the exercising woman. A significant body of evidence supports the hypothesis that healthy women can perform acute exercise of moderate intensity and duration without jeopardizing fetal well-being. Compiled studies also suggest that maximal oxygen uptake (VO2max, L.min-1) and the work rate at the onset of blood lactate accumulation (OBLA) are not significantly altered during the course of a normal pregnancy. However, some evidence suggests that maximal anaerobic power may be reduced. More information is needed on maternal cardiorespiratory function, carbohydrate metabolism, and acid-base balance at exercise intensities above OBLA and on fetal adaptability to strenuous maternal exercise. Recent studies support the view that moderate fitness conditioning can augment maternal metabolic and cardiopulmonary capacities without altering fetal development or pregnancy outcome. Implications of recent scientific studies for the design of aerobic exercise programs for pregnant women are discussed.