In this article the effect of exercise on pregnancy-related back pain is reported. Sixty five pregnant volunteers were included in the study, of whom 27 were enrolled in exercise classes designed according to Canadian guidelines, and 38 acted as sedentary controls. Back pain and functional limitations were assessed every 4 weeks during pregnancy and 4 months postpartum by questionnaires. No significant differences in back pain or functional limitations reported by the two groups were found. The biomechanical factors measured in this study, weight, back posture, and laxity, were unable to explain most of the variation in pain reported.RelevanceThis study showed that fitness classes for pregnant women designed according to Canadian guidelines had no detectable effect on back pain during pregnancy. If prevention of pregnancy-related back pain is sought through exercise, design of fitness programmes for pregnant women should be reconsidered.