A Cochrane review included 14 studies involving a total of 1 014 women. Of the 9 studies reporting on physical fitness, 6 reported significant improvement in physical fitness in the exercise group, although inconsistencies in summary statistics and measures used to assess fitness prevented quantitative pooling of results. 11 studies reported on pregnancy outcomes. A pooled increased risk of preterm birth (RR 1.82, 95% CI 0.35 to 9.57) with exercise, albeit statistically nonsignificant, does not cohere with the absence of effect on mean gestational age (MD +0.10, 95% CI –0.11 to +0.30 weeks), while the results bearing on growth of the fetus are inconsistent. One small trial reported that physically fit women who increased the duration of exercise bouts in early pregnancy and then reduced that duration in later pregnancy gave birth to larger infants with larger placentas.
Comment: The quality of evidence is downgraded by imprecise results (limited study size for each comparison).