BACKGROUND Physiological responses of the fetus (especially increase in heart rate) to single, brief bouts of maternal exercise have been documented frequently. OBJECTIVES The objective of this review was to assess the effects of advising healthy pregnant women to engage in regular (at least two to three times per week) aerobic exercise on physical fitness, labour and delivery, and the outcome of pregnancy. SEARCH STRATEGY The Cochrane Pregnancy and Childbirth Group trials register was searched. SELECTION CRITERIA Acceptably controlled comparisons of prescribed aerobic exercise programmes. DATA COLLECTION AND ANALYSIS One reviewer assessed trial quality and extracted data. Study authors were contacted for additional information. MAIN RESULTS Five trials involving 142 women were included. The trials were not of high methodologic quality. Three trials reported significant improvement in physical fitness in the exercise group, although differences in measures used to assess fetuses prevent a quantitative pooling of results. Two small trials reported nonsignificant results on pregnancy outcomes, but apart from a reassurring absence of effect on mean gestational age [+0.3 (-0.2 to +0.9) weeks], these results allow exclusion of only extremely large effects. REVIEWER'S CONCLUSIONS Regular aerobic exercise during pregnancy appears to improve (or maintain) physical fitness. Available data are insufficient to exclude important risks or benefits for the mother or infant.