Maternal Underweight is Associated with Lower Fracture Risk after Pregnancy: A Nationwide Register-Based Study in Finland

Lower body mass index (BMI) is associated with a higher risk for osteoporotic fractures in the postmenopausal population. However, in the fertile-aged population, the association between BMI and risk for fracture is not well studied. Our aim, therefore, is to investigate whether lower BMI (<18.5 kg/m 2 ) affects the risk for fractures requiring hospitalization after delivery in fertile-aged women when compared to women of normal weight (18.5 ≤ BMI < 25 kg/m 2 ). In this nationwide registry-based cohort study, all pregnancies were gathered from the National Medical Birth Register. The data were linked with data from the Care Register for Health Care, which includes information of all fractures leading to hospitalization or treated as outpatients for fertile-aged females (15–49 years), for the period 2004–2018. The annual rate of pregnancies with maternal pre-pregnancy underweight in a high-income country was calculated. Cox regression was used to calculate adjusted hazard ratios (aHR) of lower BMI for the risk for fracture within 5 years after delivery. The association between the risk of fracture and continuous BMI was assessed using logistic regression and presented with adjusted odds ratios (aORs) with 95% CIs. In total, 20,784 women were included in the underweight group and 344,753 in the normal weight group. Women in the underweight group had a lower overall risk for any fractures during the 5-year follow-up (aHR 0.75, CI 0.61–0.94). The odds for all fractures (aOR 1.07, CI 1.04–1.09 per BMI-unit upwards for all fractures) increased as pre-pregnancy BMI increased. Despite previous findings of a higher risk for fractures for underweight patients in the older population, the risk for fractures after pregnancy was lower among fertile-aged women with lower BMI.