Body mass index trajectories and age at menopause in a British birth cohort

OBJECTIVE: This study investigates the influence of body mass index (BMI) at ages 15, 20, 26, 36, and 43, and of BMI trajectories from 20 to 36 years on the timing of menopause and hormone therapy (HT) use until age 57 years. METHODS: A nationally representative British cohort of 1583 women born in March 1946 with prospective data across the life course. RESULTS: By age 57, a total of 695 women had experienced natural menopause while 431 women had started HT prior to menopause. Cox regression models indicated no significant associations between BMI at any age, or BMI trajectory, and timing of natural menopause. At every age BMI was strongly (p ≤ 0.01) and linearly associated with age at HT use and BMI from 26 years onwards was associated with age at first event (menopause or HT use). Decreasing BMI was associated with earlier HT use at all ages. These associations were not accounted for by parity, cigarette smoking or childhood and adult social class. CONCLUSION: BMI across the reproductive lifespan did not influence age at menopause to an extent that would be clinically relevant for postmenopausal health. Lower BMI at all ages and underweight trajectory were related to an earlier start of HT. Further studies are required to understand whether such relationships are due to underweight women experiencing menopause earlier (and because of menopausal symptoms starting HT earlier) than heavier women, or having behavioural characteristics related to earlier HT use, independent of menopause.