The realities are that women's exercise levels drop by 20 percent after they become mothers, Schmitz says. This change to a more sedentary lifestyle is a significant health hazard, notes the AHA, which puts inactivity on par with smoking, high blood cholesterol and high blood pressure as a major risk factor for heart disease.

As many as 250,000 deaths each year--about 12 percent of total deaths--are attributable to lack of regular physical activity, the association says.

Schmitz found that the biggest drop in exercise levels occurred after women became mothers for the first time; there was little if any change with subsequent children. The decline was similar for both married and single mothers. In contrast, men's exercise levels didn't change after they became parents, regardless of their marital status.

"This suggests that women are doing the lion's share of child care, whether the father is present or not," Schmitz says.

To reverse this trend toward sedentary habits, Schmitz says, "moms must take time to exercise, and programs should be designed that make it convenient for them to enjoy an active lifestyle."