et al. Physical exercise during pregnancy and the risk of preterm birth: a study within the Danish National Birth Cohort. Am J Epidemiol. 2008;167: 859-866. Objective: To examine the relation between physical exercise during pregnancy and the risk of preterm birth in a large cohort of women. Design: Prospective, observational, cohort study. Setting: Nationwide study with recruitment through general practitioners, during the period 1996 to 2002. Participants: Pregnant women were recruited into the Danish National Birth Cohort. Approximately 50% of Danish general practitioners collaborated, and 60% of the invited women agreed to participate. After exclusions (pregnancy terminated before 22 weeks of gestation; or interview took place after 37 weeks of gestation) 89 196 women completed the first computer-assisted telephone interview at a median gestational age of 16.3 completed weeks, and 82 965 women (93%) completed a second interview at a median gestational age of 31.1 completed weeks. Assessment of risk factors: Both interviews included questions on the respondents' physical exercise during the current pregnancy. Type, duration, and frequency of exercise were recorded for each activity. Physical exercise was summarized into 6 categories of hours per week, from 0 h/wk to .5 h/wk. The type of preferred exercise was categorized as swimming, low impact activities, high impact activities, fitness training, bicycle riding, horseback riding, and miscellaneous. Changes in physical activity from interview 1 to interview 2 were noted. Potential confounders such as variables within the pregnancy, the health and reproductive history of the mother, and her lifestyle, social, and work factors were assessed. Main outcome measures: The main outcome measure was the relation of preterm birth to the mother's physical activity. Gestational age of the birth was obtained from birth record data reported to the National Patient Registry in Denmark. Preterm birth was defined as delivery (live or stillbirth) after 22 weeks and before 36 completed weeks of gestation. Preterm births were subcategorized as 22-27, 28-31, and 32-36 completed weeks of gestation.Main results: Most of the women (63%) did not do any kind of physical exercise at the time of the first interview. The proportion increased to 70% at the second interview. Among the active women, swimming, low-impact activities and bicycling were the preferred activities at both interviews. Exercising .2 h/wk was uncommon, and in late pregnancy approximately half the active women exercised for #1 h/wk. The number of preterm births was 4279 (4.9% of singleton pregnancies). Hazard ratios (HR) for preterm birth were lower for women who did any exercise compared with those who did none (unadjusted HR, 0.86 and adjusted HR, 0.82; 95% CI, 0.76-0.88). Although HR for preterm birth were significantly lower for women who exercised .0 to #1 h/wk and .1 to #2 h/wk than for the nonexercisers, there was no trend for an association between fewer preterm births and increasing exercise when the no-exe...
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