Dr. Jennifer Lovejoy of the University of Washington in Seattle and Dr. Amanda Sainsbury-Salis of the Garvan Institute of Medical Research in Sydney say many women get confused when they start to gain weight during menopause, because their eating habits haven't changed.

"What the research shows clearly is that menopause causes a dramatic and sudden reduction in physical activity levels. Just prior to menopause, women halve their amount of activity compared to pre-menopause levels," the researchers say in a statement.

"We don't know exactly why menopausal women stop moving as much. But we know it's not because of their age and the lifestyle constraints happening at that time of life. Research suggests it's directly related to the lack of estrogen, which has dramatic effects on signals the brain sends to the body."

However, it's been shown that women who maintain or increase their level of physical activity during menopause tend to not gain weight, the researchers say.

The findings are published online in the December issue of Obesity Reviews.