Reproductive factors, bone mineral density, and fracture history: the NORA study

Abstract

Objective: To examine the associations of elements of reproductive history with peripheral bone mineral density (BMD) t scores and fracture history after age 45 years. Methods: Data on risk factors and reproductive history were collected from 47,050 women who had not had hysterectomies. The key variable of interest was the duration of endogenous estrogen exposure (difference between the ages of menarche and menopause). Other variables examined included parity, lactation, oral contraceptive (OC) use, age of mother at birth of first- and last-born children, and postmenopausal (PM) estrogen supplementation. We estimated the separate and collective effects of these variables on BMD and fracture risk using bivariate analyses and regression models adjusted for age, ethnicity, and level of education. Results: Using women with less than 30 years of endogenous estrogen exposure for comparison, t scores increased 0.12, 0.25, 0.41, and 0.53, and the odds ratios for fracture were 0.92, 0.87, 0.76, and 0.76 among women with 30–34, 35–39, 40–44, and 45+ years of endogenous estrogen, respectively. Oral contraceptive use, parity, and PM estrogen use also had significant positive effects on BMD. Conclusion: The study provides evidence that the duration of endogenous estrogen exposure has a beneficial effect on BMD and lowers fracture risk, even after adjusting for BMD.