Preconception underweight impact on postnatal osteoporotic fracture: a retrospective cohort study using Japanese claims data.

Background: Undernutrition and underweight are osteoporosis risk factors. Therefore, improving the health of underweight young women in Japan is an important medical issue. However, few studies have evaluated the association between being preconception underweight and postnatal osteoporotic fractures in young women.

Methods: This retrospective cohort study used a Japanese nationwide claims database (JMDC Inc.) to evaluate the effect of preconception underweight on the incidence of osteoporotic fracture within two years after delivery. Data from 16,684 mothers who delivered their first singleton babies between January 2006 and December 2020 were analysed. The combination of disease codes of fractures at sites associated with osteoporosis and medical procedures for fractures was defined as the incidence of osteoporotic fractures, whereas the body mass index (BMI) recorded 12-36 months before delivery was used as the exposure. We estimated the incidence of osteoporotic fractures by BMI category using a Kaplan-Meier curve and examined the fracture risk using Cox hazard regression analyses.

Results: Fifty-one women (0.31%) were affected by osteoporotic fractures within two years of delivery. More than 80% of these were rib fractures, and approximately 65% of fractures occurred after the first year postpartum. Preconception underweight (BMI < 18.5 kg/m) was significantly associated with the incidence of postpartum osteoporotic fractures. There was no significant association between low BMI and postnatal fractures, as analysed via multiple categorical logistic regression analysis.

Conclusion: Appropriate control of preconception weight might be critical to improving the postpartum quality of life, subsequent bone health, and neonatal care environment.