Background <p>Postmenopausal osteoporosis (PMOP) has high morbidity and devastating complications. However, few studies explored the modifying role of different types of physical activity (PA) in the associations between the number of pregnancies as well as livebirths and the odds of PMOP. This study aimed to investigate the associations of the number of pregnancies and livebirths with PMOP odds.</p> Methods <p>This study used the baseline of the China Multi-Ethnic Cohort study, and a total of 27,459 postmenopausal women were recruited. Bone mineral density and quantitative ultrasound index were measured by quantitative ultrasound bone densitometer. Number of pregnancies and number of livebirths were collected by questionnaires. PA was classified into occupational, transportation, housework, and leisure PA. Logistic regression and multiple linear regression were used to analyze the associations between the number of pregnancies and livebirths and PMOP odds. The modifying effects of PA and its domain-specific types in the associations were analyzed by interaction analysis.</p> Results <p>A total of 1,539 (5.6%) participants had PMOP. Each additional pregnancy and each additional livebirth were associated with a 5% (OR [95% CI]: 1.05 [1.02, 1.09]) and 21% (1.21 [1.15, 1.28]) higher odds of PMOP, respectively. High levels of housework PA attenuated the adverse effect of livebirths on the PMOP odds (<i>P</i> for interaction &lt; 0.05, <i>q</i> &lt; 0.05). Associations between the number of pregnancies and livebirths and the odds of PMOP were observed among women of older age, with earlier age at menopause, later age at menarche, and longer total breastfeeding duration.</p> Conclusions <p>An increased number of pregnancies and livebirths was positively associated with the odds of PMOP. However, high levels of housework PA attenuated the adverse effect of livebirths on the PMOP odds, suggesting that an active lifestyle is associated with more favorable bone health status.</p>