Preterm birth (PTB) is the major cause of perinatal mortality in the United States. In the past, pregnant women have been recommended against exercise because of presumed risks of PTB. Physical activity has been theoretically related to PTB as it increases the release catecholamines, especially norepinephrine, which might stimulate myometrial activity. Conversely, exercise may reduce the risk of PTB by other mechanisms such as decreased oxidative stress or improved placenta vascularization. Therefore, the safety of exercise regarding PTB and its effects on gestational age at delivery remain controversial.To evaluate the effects of exercise during pregnancy on the risk of PTB.MEDLINE, EMBASE, Web of Sciences, Scopus, ClinicalTrial.gov, OVID and Cochrane Library were searched from the inception of each database to April 2016.Selection criteria included only randomized clinical trials (RCTs) of pregnant women randomized before 23 weeks to an aerobic exercise regimen or not. Types of participants included women mainly of normal weight with uncomplicated, singleton pregnancies without any obstetric contraindication to physical activity. The summary measures were reported as relative risk (RR) or as mean difference (MD) with 95% confidence intervals (CI). The primary outcome was the incidence of PTB <37weeks.Of the 2,059 women included in the meta-analysis, 1,022 (49.6%) were randomized to the exercise group and 1,037 (50.4%) to the control group. Aerobic exercise lasted about 35-90 minutes 3-4 times per week. Women who were randomized to aerobic exercise had a similar incidence of PTB<37 weeks (4.5% vs 4.4%; RR 1.01, 95% CI 0.68-1.50) and a similar mean gestational age at delivery (MD 0.05 week, 95% CI - 0.07 to 0.17) compared to controls. Women in the exercise group had a significantly higher incidence of vaginal delivery (73.6% vs 67.5%; RR 1.09, 95% CI 1.04-1.15) and significantly lower incidence of cesarean delivery (17.9% vs 22%; RR 0.82, 95% CI 0.69-0.97) compared to controls. The incidence of operative vaginal delivery (12.9% vs 16.5%; RR 0.78, 95% CI 0.61-1.01) was similar in both groups. Women in the exercise group had a significantly lower incidence of gestational diabetes mellitus (2.4% vs 5.9%; RR 0.41, 95% CI 0.24-0.68) and significantly lower incidence of hypertensive disorders (1.9% vs 5.1%; RR 0.36, 95% CI 0.19-0.69) compared to controls. No differences in low birth weight (5.2% vs 4.7%; RR 1.11, 95% CI 0.72-1.73) and mean birth weight (MD -10.46 grams, 95% CI -47.10 to 26.21) between exercise group and controls were found.Aerobic exercise for 35-90 minutes 3-4 times per week during pregnancy can be safely performed by normal-weight women with singleton, uncomplicated gestations, as this is not associated with an increased risk of PTB or with a reduction in mean gestational age at delivery. Exercise was associated with a significantly higher incidence of vaginal delivery and a significantly lower incidence of cesarean delivery, with a significantly lower incidence of gestational diabetes mellitus and hypertensive disorders, and therefore should be encouraged.