Study Overview
Objective. To evaluate if exercise during pregnancy has an effect on the risk of preterm birth.
Design. Systematic review and meta-analysis of randomized controlled trials.
Study selection . The authors followed the protocol for conducting meta-analyses recommended by the Cochrane Collaboration. MEDLINE, EMBASE, Web of Science, Scopus, ClinicalTrials.gov, OVID, and the Cochrane Library were searched from the inception of each database to April 2016. Selection criteria included randomized clinical trials that examined the effect of aerobic exercise on preterm birth. Keywords included exercise or physical activity and pregnancy and preterm birth or preterm delivery. Studies were included only if women were randomized to an aerobic exercise program prior to 23 weeks, participants had uncomplicated singleton pregnancies and no contraindication to exercise, and preterm birth was an outcome.
Nine studies met the inclusion criteria and were included in the meta-analysis. The quality of included studies was good overall, with most studies having low risk of selection or attrition bias and low or unclear risk of reporting bias. Most of the studies did not include blinding of participants and research personnel or of the outcome assessment. Sample sizes ranged from 14 to 697, with 2 studies with < 100 participants, 3 with 100 to 200 participants, and 3 with 290 to 687 participants. All of the women randomized to the experimental group began an exercise program by 22 weeks’ gestation. The types of physical activity used in the experimental group included strength and flexibility training, cycling, stretching, resistance, dance, joint mobilization, walking, and toning. Participants engaged in the activity for 35 to 90 minutes (mean, 57 minutes) 3 times a week in 8 studies and 4 times a week in 1 study. The intensity of the aerobic activities ranged from less than 60% to less than 80% of age-predicted maximum heart rate. Participants in 3 control groups were explicitly told not to engage in exercise while those in the others were neither encouraged or discouraged from doing so.
Main outcome measure. Incidence of preterm birth (birth prior to 37 weeks’ gestation).
Main results. A total of 2059 women were included in the meta-analysis, with 1022 in the exercise group and 1037 in the control group. The incidence of preterm birth was similar in the experimental and the control groups (4.5% vs 4.4% respectively, 95% confidence interval [CI], –0.07 to 0.17). The mean gestational age at delivery was also similar, with a mean difference of 0.05 (95% CI, –0.07 to 0.17). Women in the exercise group had a decreased risk of cesarean delivery (0.82%), with 17.9% having a cesarean delivery compared to 22% in the control group ( 95% CI, 0.69 to 0.97).
Conclusion. Exercise during pregnancy in women with singleton, uncomplicated pregnancy is not associated with increased risk of preterm delivery. Additionally, it is associated with a decreased risk of cesarean delivery.
Commentary
Preterm birth accounts for most perinatal deaths in the United States and places surviving infants at risk for serious short- and long-term health problems [1]. Though the rate of preterm births in the United States has been slowly declining in recent years, at 9.57% it continues to be one of the highest among high-income countries [2]. Determining factors that contribute to incidence of preterm birth is critical to reducing this unacceptably high rate. According to the authors of this meta-analysis, the role of exercise related to preterm birth remains controversial due to past beliefs that the increased release of catecholamines during exercise would stimulate myometrial activity and ongoing concerns about possible adverse effects. The health benefits of regular exercise are well-known, including in pregnancy where it has been shown to lower the risk of gestational diabetes and preeclampsia.
Researchers have investigated exercise during pregnancy in earlier reviews; however, this appears to be the first with both preterm birth as the primary outcome and an adequate number of clinical trials in the sample. Prior reviews that examined the effects of exercise on preterm birth, either specifically or as one of a number of pregnancy outcomes, included only 3 to 5 studies pertaining to preterm birth [3–5].
The strengths of this review were the low statistical heterogeneity and high quality of the included studies, lack of publication bias, and the large sample of 2059 participants. As noted by the authors, however, lack of stratification by body mass (underweight, overweight, obese), differences in the types and intensity of exercise among interventions, as well as possible differences in adherence may have affected outcomes. In addition, in 6 studies women in the control group were not specifically instructed to refrain from exercise and there is no information about their exercise habits. The risk of contamination bias exists because some of these women may have engaged in a regular program of exercise. However, considering that levels of regular exercise in pregnant women are low, it is unlikely that this would occur at a rate that would have a significant effect on the outcomes [6].
Applications for Clinical Practice
The results of this meta-analysis provide strong support for the American College of Obstetrics and Gynecology recommendation that women with uncomplicated pregnancies be encouraged to engage in moderate-intensity exercise 20 to 30 minutes per day during pregnancy [7]. Clinicians should advise all women with uncomplicated singleton pregnancies and no medical contraindications to engage in regular aerobic and strength-conditioning exercise throughout their pregnancy.
—Karen Roush, PhD, RN