A 10-year study of young adult men and women found that parenthood led to women spending less of their leisure time in physical activities.

Men's leisure time physical activity was unaffected by becoming fathers.

The research was presented to the American Heart Association's epidemiology and prevention meeting on Friday.

Dr Kathryn Schmitz, of the University of Minnesota, said: "This puts women at greater risk for heart disease and other diseases associated with a sedentary lifestyle.

"The biggest difference occurred when they became first-time parents.

"There wasn't any additional change in physical activity when going from one to two children."

Dr Schmitz said health professionals should develop exercise programmes for new mothers that enabled them to work out with their children present.

Physical inactivity doubles the risk of coronary heart disease, the cause of heart attacks.

The doubling of risk is comparable to the effects of other major risk factors, such as high blood cholesterol, high blood pressure or smoking.

The researchers initially assumed that mothers who were the sole caregivers would have a more dramatic drop in physical activity than married mothers.

However, that was not the case.

Dr Schmitz said: "We were thinking if the woman was a single parent there would be just that much more of a load.

"But it didn't matter whether the woman was married or not."

Dr Schmitz said the simple reason might be that women may not have the time to exercise.

"We can only speculate that the reason women are reporting less leisure-time physical activity overall than men is that they are spending more time caring for children."

Major study

The data were derived from the CARDIA (Coronary Artery Risk Development in Young Adults) study, which has followed men and women, aged 18 to 30, for 10 years to track both health and behavioral risk factors of heart disease.

The researchers measured physical activity in terms of exercise units (EUs).

For instance, 50 EU are the equivalent of jogging two hours weekly for six months of the year.

The results showed that white women, who averaged about 400 EU annually before parenthood, dropped an average of 56 EU after parenthood.

Black women, who averaged 278 EU before becoming mothers, dropped 26 EU annually.

The most commonly reported types of leisure-time physical activity among the study participants were walking, non-strenuous leisure sports and home exercise (calisthenics).

The British Heart Foundation said the study appeared to support the need to assess the amount of activity carried out by parents, especially first time mothers.

However, in a statement, the BHF added: "It would appear that this research does not take into consideration the various forms of activity involved in caring for a new child, such as walking the baby in the pram, lifting the child, housework and swimming.