The American College of Obstetricians and Gynecologists recommends that women with low-risk pregnancies participate in moderate- intensity exercise during their pregnancy. Currently, only 15.1% of pregnant women exercise at the recommended levels, which is significantly lower than the general population's 45%. One potential reason is that exercise during pregnancy is perceived as risky. In this article, the authors provide a critical review of the literature examining the effect of exercise on pre-eclampsia, gestational diabetes, weight gain, labor and birth, and other issues associated with pregnancy. Overall, the evidence indicates that exercise during pregnancy is safe and perhaps even reduces the risk of preeclampsia and gestational diabetes. The evidence for weight gain and labor and birth (rates of cesarean sections, duration of labor) is mixed. Unfortunately, much of the research examining exercise during pregnancy is observational, and the few randomized controlled trials that do exist are small and inadequately powered. Taken together, given the potential benefits of exercise during pregnancy and the lack of evidence for harmful effects on the mother and newborn, practitioners should encourage their healthy pregnant patients to exercise. Practical guidelines for recommending exercise to pregnant women are presented.
And the Summary:
Research indicates that exercise likely has a positive effect on maternal outcomes among pregnant women.  Observational studies suggest that exercise during pregnancy is related to various health benefits, including reduced rates of preeclampsia, hypertensive disorders of pregnancy, gestational diabetes, cesarean sections, low back pain, anxiety, nausea, heartburn, insomnia, and leg cramps. [14,43,49,56] In addition, some observational studies have linked exercise during pregnancy to a reduced risk of excessive weight gain, [29,33,46] although the findings across studies are somewhat conflicting. [62,63] Studies also indicate that newborn birth weight does not significantly differ between women who exercise and women who do not exercise during pregnancy. [34-36,62]