The female gender and multiparity are the two most important positive correlates of cholesterol gallstone disease. Pregnancy represents the period of time when the 'lithogenic' pressure on a woman is the highest. Biliary sludge is a precursor stage of gallstones. The investigators studied the etiological factors associated with the development of sludge and stones during pregnancy, and their early results suggest that it is inversely related to physical activity. They also found that being overweight, a known risk factor for gallstone disease, is associated with high blood leptin levels. In addition, the risk associated with high leptin levels is partially mitigated by physical activity.