Abstract Background: Physiological responses of the fetus (especially increase in heart rate) to single, brief bouts of maternal exercise have been documented frequently, suggesting that maternal exercise might have beneficial or adverse effects on pregnancy outcome. 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 and researchers in the field were contacted. Date of last search: January 2002. Selection criteria: Acceptably controlled comparisons of prescribed aerobic exercise programs. Data collection and analysis: One reviewer assessed trial quality and extracted data. Study authors were contacted for additional information. Main results: Ten trials involving 688 women were included. The trials were not of high methodologic quality. Five trials reported significant improvement in physical fitness in the exercise group, although differences in measures used to assess fitness prevent a quantitative pooling of results. Six trials reported on pregnancy outcomes. A statistically significant increased risk of preterm birth (relative risk (RR) = 2.29 [95% confidence interval 1.02 to 5.13]) in the exercise group is inconsistent with the absence of effect on mean gestational age [+0.0 (−0.4 to +0.4) weeks] and may have been biased by post-randomization exclusion of women with preterm labor. Results bearing on fetal growth (birthweight, length, and head circumference) were highly variable. One trial found improvements among exercising women in several aspects of self-reported body image, including muscle strength, energy level, and body build. Reviewers’ conclusions: Regular aerobic exercise during pregnancy appears to improve (or maintain) physical fitness and body image. Available data are insufficient to infer important risks or benefits for the mother or infant. Citation: Kramer MS. Aerobic exercise for women during pregnancy (Cochrane Review). In: The Cochrane Library, Issue 3, 2003. Oxford: Update Software.