The interaction between maternal endurance exercise at or above a minimal conditioning level, prior to and during pregnancy, and pregnancy outcome was examined prospectively in our obstetrical populace using epidemiologic techniques. Over a 3-month interval all women registering for antepartum care were interviewed. Those planning to continue exercise during pregnancy were re-interviewed between the 28th and 34th gestational weeks. A detailed review of clinical records was used for outcome assessment. Women who continued endurance exercise at or near preconceptual levels during pregnancy gained less weight (-4.6 kg), delivered earlier (-8 d), and had lighter-weight offspring (-500 g) than those who stopped exercising prior to the 28th week. The latter group gained 2.2 kg more weight but delivered similar birthweight infants at a similar gestational age as their sedentary controls.