OBJECTIVE The effects of high- and medium-intensity exercise on the fetus and on the onset and length of labor, birth weight, and Apgar score were studied in healthy athletes who performed a high level of exercise before conception. METHODS Forty-two women were recruited to the study by newspaper ads and through acquaintances. They elected to follow either a high- or a medium-intensity exercise program throughout pregnancy until 6 weeks after delivery. Documentation of their intensity of activity before conception (retrospectively), during pregnancy, and after delivery was obtained. RESULTS There were no differences between the high- and medium-intensity exercise group in duration of labor, birth weight, or 1- and 5-minute Apgar scores. The higher level of exercise correlated with a significantly greater maternal weight gain during pregnancy and significantly earlier onset of labor for those women who gave birth to girls but not for those who gave birth to boys. CONCLUSIONS Our results indicate that healthy and well-conditioned women may take part in exercise during pregnancy without compromising fetal growth and development as judged by birth weight or complicating the course of pregnancy or labor.