NEW YORK (Reuters Health) Mar 15 - Women who are physically active before pregnancy are less likely to develop gestational diabetes mellitus (GDM), and the GDM risk also rises with the amount of pre-pregnancy television viewing, a new study shows.

While smaller studies have suggested that exercise can reduce GDM risk, the current study is among the largest and best-designed that has examined this relationship to date, lead investigator Dr. Cuilin Zhang of the Harvard School of Public Health in Boston told Reuters Health. The study also looks at physical activity in greater detail, she added.

Dr. Zhang and her colleagues evaluated at 21,765 women participating in the Nurses Health Study II who had at least one singleton pregnancy between 1990 and 1998. A total of 1428 GDM cases were identified. The women had completed validated questionnaires assessing their level of physical activity and sedentary behavior.

After the researchers controlled for body mass index, diet and other potential confounding factors, they found that women in the highest quintile for vigorous physical activity were 23% less likely to develop GDM than those who were the least active.

Women who walked briskly but did not perform vigorous exercise were 34% less likely to develop GDM than those who walked at an easy pace, the researchers report in the March 13th issue of the Archives of Internal Medicine. Walking briskly for more than 30 minutes daily or climbing 15 flights of stairs every day also lowered GDM risk for women who did not exercise vigorously.

The researchers also found that women who watched at least 20 hours of television each week and did not exercise vigorously were 2.3 times more likely to develop GDM than active women who watched less than 2 hours of TV a week.

Most studies of exercise have looked at its effects on chronic disease in middle-age and elderly women, Dr. Zhang noted. Given that GDM is a risk factor for developing type 2 diabetes, she said, the findings underscore the importance of physical activity for younger women as well. There's also evidence that GDM can increase a child's later risk of obesity and diabetes, she added.

"From the public health view, it is important for women of reproductive age to keep an active lifestyle," she said.

Arch Intern Med 2006;166:543-548.