OBJECTIVETo evaluate the influence of exercise on maternal and perinatal outcome in a low-risk healthy obstetric population.METHODSWe conducted a prospective observational study of low-risk healthy women exercising during their pregnancy. An extensive questionnaire collected antepartum, intrapartum, and postpartum patient information on 750 women. The women were divided into four groups based on exercise level during pregnancy.RESULTSThere were no differences among groups for maternal demographic characteristics, antenatal illnesses, stress, social support, or smoking. Heavily exercising women were older (P= .042), had higher incomes (P= .001), and were exercising more at conception (P= .001). Women who did more exercise were more likely to need an induction of labor (P= .033, relative risk 1.84, 95% confidence interval 1.05, 3.20), induction or augmentation with oxytocin (P= .015, relative risk 1.53, 95% confidence interval 1.19, 1.97), and had longer first-stage labors (P= .032) resulting in longer total labors (P= .011). The difference in the length of first-stage labor was even greater if the no-exercise group was compared with the strongly exercising group (P= .009, relative risk 1.38, 95% confidence interval 0.16, 2.60). Fewer umbilical cord abnormalities (P= .034) were observed with exercise, but exercising women had more colds and flu (P= .008). Heavily exercising women had smaller infants (mean difference 86.5 g) compared with sedentary women.CONCLUSIONExercise in working women is associated with smaller babies, increased number of inductions and augmentations of labor, and longer labors. Colds and flu are more frequent in exercising women.