Leisure time physical exercise during pregnancy and the risk of miscarriage: a study within the Danish National Birth Cohort

Objective: To examine the association between leisure time physical exercise during pregnancy and the risk of miscarriage. Design: Prospective study with elements of retrospective data collection. Setting: Denmark 1996-2002. Population: A total of 92 671 pregnant women enrolled in the Danish National Birth Cohort and interviewed subsequently. Methods: Data on exercise during pregnancy and potential confounders were obtained through computer-assisted telephone interviews either during pregnancy or after an early miscarriage. Outcome of pregnancy was identified by register linkage. Using Cox regression analysis, we estimated the hazard ratio (HR) of miscarriage according to weekly amount of exercise and the type of exercise. The HR was estimated for <11, 11-14, 15-18, and 19-22 weeks of gestation, respectively. Main outcome measures: Miscarriage, defined as fetal loss before 22 completed weeks of gestation. Results: A stepwise increasing relation was found between amount of exercise and risk of miscarriage, where risk of miscarriage increased by amount of exercise up to HR = 3.7 (95% CI 2.9-4.7) for women who exercised more than 7 hours per week compared with nonexercisers. Particularly 'high-impact exercise' was associated with an increased risk of miscarriage. No association was seen between exercise and risk of miscarriage after 18 weeks of gestation. Conclusions: This study suggests that exercise early in pregnancy is associated with an increased risk of miscarriage. The results should, however, be interpreted cautiously as potential bias arising from retrospective data collection may explain part of the association.