OBJECTIVE: This study examined the question of whether vigorous exercise undertaken by recreational exercisers across pregnancy, defined in two ways, were associated with reduced infant birthweight and gestational age at birth. METHODS: A prospective approach was implemented. A total of 148 pregnant women participated. Average intensity duration and frequency of vigorous exercise reported were examined and compared with two existing definitions of vigorous exercise. Participants completed questionnaires (including retrospective reports on 3 months prepregnancy) and an exercise diary at 16-23 weeks pregnancy, 24-31 weeks pregnancy and 32-38 weeks pregnancy, and at 7 to 14 days post-partum a birth outcomes questionnaire was completed. RESULTS: There were no significant differences between exercise groups for birthweight and gestational age at birth. CONCLUSIONS: There was no evidence that the intensity duration and frequency of vigorous exercise were associated with significant reductions in mean birth outcomes for the infants of women who participated in the study. Replication in a large, more diverse sample is recommended.