This study was designed to test the hypothesis that vigorous aerobic exercise during both the periconceptional period and early pregnancy increases the incidence of abnormal early pregnancy outcome. Exercise performance was prospectively monitored before and during pregnancy in 47 recreational runners, 40 aerobic dancers, and 28 physically active, fit controls. Pregnancy was diagnosed by an early test for beta-subunit human chorionic gonadotropin and viability was confirmed by ultrasonography at 40 days' conceptional age. Spontaneous abortion occurred in 19% of the pregnancies. The incidence was 17% in the runners, 18% in the aerobic dancers, and 25% in the controls. At term, one congenital abnormality was detected in each of the three groups. Late pregnancy events, potentially related to abnormalities of placentation, were limited to two cases of mild pregnancy-induced hypertension. We conclude that, in physically fit women, continuation of these types of aerobic activity at intensities between 50% and 85% of maximum during the periconceptional period and early pregnancy does not appreciably alter early pregnancy outcome.