In utero exposures have been proposed as possible determinants of later disease risk. Given that a later age at menarche is a breast cancer risk factor, and that higher childhood physical activity has been associated with a later menarcheal age, it is possible that a pregnant mother's activity may also influence this outcome. The purpose of this study was to determine if a mother's physical activity during pregnancy is related to their daughter's menarcheal age. Participants of the Nurses' Health Study II reported their age at menarche to the nearest year, whereas their mothers (n=33,016) completed surveys regarding their health and lifestyle habits during their pregnancy with their daughters. Mothers reported their home, occupational, and leisure-time physical activities, as well as the activity of their daughters at ages 5 to 10 years. Using multiple linear regression analysis with adjustment for specific covariates including daughter's childhood body size, neither home nor occupational activity alone were associated with age at menarche of the daughter, but there was a direct association with leisure-time physical activity (P(trend)<0.001). Compared with women inactive in their leisure-time, women who were highly active had daughters with menarche 1.1 (95% confidence interval, 0.3-1.9) months later. Using a composite variable of both home and leisure-time activity, daughters of women who were highly active at home and in their leisure-time had daughters with menarche 3.1 (95% confidence interval, 0.4-5.9) months later than those who were highly inactive in both. Physical activity during pregnancy may be associated with a modest delay in menarcheal age in offspring.