Using the Danish National Birth Cohort database, Mette Juhl, of the National Institute of Public Health, in Copenhagen, and colleagues analysed physical activity information reported by 87 232 pregnant women between 1996 and 2002.
Overall, about one-third of the women said they participated in physical exercise, such as swimming, or low-impact leisure time activity such as aerobics, dancing, walking or hiking, bicycling, and yoga during early pregnancy.
Most of these women engaged in such activities for one to two hours per week, and somewhat fewer reported the same during later pregnancy, the investigators note.
Juhl and colleagues found that women who engaged in swimming or low-impact physical activities while pregnant were less likely to give birth before term than women who did not exercise during pregnancy.
Neither the type of exercise nor the level of exercise altered this association.
In a report in the American Journal of Epidemiology, Juhl and colleagues say their findings confirm the physical activity recommendations from health authorities in Denmark, Norway, Great Britain, and the United States that suggest pregnant women exercise at levels similar to non-pregnant women.