Associations between reproductive factors and frailty in middle-aged and older women: evidence from the China health and retirement longitudinal study. - Xian, Xiaobing; Xiang, Jie; Cao, Shiwei; Tang, Wanting; Wu, Yao; Li, Jiaxia; Ren, Shiqin; Wang, Yuanyuan; Shen, Kun (Journal of Health, Population & Nutrition, 2025)