Non-linear associations between parity and long-term risk of osteoporosis

Summary Our findings suggest that although pregnancy and lactation are periods of physiological calcium loss, higher parity is not associated with an increased risk of postmenopausal osteoporosis; rather, it appears to confer a long-term protective effect. Purpose Osteoporosis is a significant public health burden. While parity induces transient bone loss, their long-term skeletal impact remains poorly understood. This study aimed to clarify the associations between parity, defined as the number of live births, and the risk of osteoporosis, osteoporotic fractures, and bone mineral density (BMD). Methods We analyzed data from 119,969 women in the UK Biobank. Associations between parity and incident osteoporosis or fractures were evaluated using unweighted and inverse probability of treatment weighting Cox proportional hazards models. Generalized linear models assessed associations with femoral neck and lumbar spine BMD. Models were adjusted for sociodemographic, genetic, lifestyle, and clinical factors. Results Compared with nulliparous women, those with a parity of 2 or 3 exhibited a significantly reduced risk of osteoporosis in the weighted fully adjusted model. In addition, parity of 3 exhibited a significantly reduced risk of osteoporotic fracture compared with the nulliparous group. This protection was evident in women ≥ 60 years and those with normal body mass index. Parity was not significantly associated with areal BMD changes after full covariate adjustment. Conclusion In women aged 40–69 years at baseline, a parity of 2 or 3 confers long-term protection against osteoporosis and osteoporotic fracture (a arity of 3), occurring independently of BMD changes. These findings support the integration of parity into clinical risk assessment tools, ultimately contributing to more effective osteoporosis prevention and a reduced fracture burden across women’s lifespan.