Background: The rate of preterm delivery has increased in recent years; however, the cause remains poorly understood. Although it is well documented that increased physical activity can decrease the risk of some diseases, the effect on preterm delivery is unclear. Previous studies on vigorous sports training have shown an unchanged or decreased risk of preterm delivery, but few studies have evaluated women who exercise less strenuously during pregnancy. To determine the effects of leisure time physical activity on preterm delivery, Hegaard and colleagues prospectively studied women with varying amounts of physical activity and their subsequent rates of preterm delivery.

The Study: Pregnant women attending an antenatal clinic in Denmark between August 1989 and September 1991 were invited to participate. Those with language barriers, pregnancy complications that could lead to pre-term delivery (e.g., cardiovascular, pulmonary or kidney disease; multiple gestation; epilepsy; diabetes; psychiatric illness), and incomplete data were excluded from the analysis. A total of 5,749 women participated, and each completed a self-administered questionnaire about her sports activities (type and time spent per week) and leisure time activities. Leisure time was described as mostly sitting; light physical activity (more than three hours per week of walking, gardening, etc.); moderate sports or heavy gardening (more than three hours per week of running, swimming, etc); or competitive sports several times weekly. Questionnaires were completed at about 16 weeks of gestation. Gestational age was determined in most of the women by a reliable last menstrual period or ultrasonography before 21 weeks of gestation. Data on possible confounders (e.g., maternal age, smoking, body mass index, marital status, education, employment status, psychological stress) were also collected.

Results: Women who classified themselves as engaging in moderate to heavy leisure activities had a significant 66 percent reduction in adjusted risk for preterm delivery (i.e., before 37 weeks of gestation), whether or not they participated in competitive sports. Women who participated in light leisure activities showed a trend toward reduced risk compared with sedentary women, but it was not statistically significant. Controlling for the confounders did not change the results. The authors propose that collecting data prospectively, early in pregnancy before many complications occurred, and at a time before the benefits of exercise were as widely known decreased bias in the self-reporting. They acknowledge, however, that because other lifestyle factors may not have been controlled for, and because the study group was overwhelmingly comprised of white, non-Hispanic women, usefulness of the results in other ethnic groups may be limited.

Conclusion: There may be an association between a sedentary lifestyle and preterm delivery in women with otherwise low-risk pregnancies. The authors conclude that continued attention to preventable causes of preterm delivery is needed.