Lifetime number of years of menstruation and risk of breast cancer

Abstract

Background: Reproductive factors and lifetime number of years of menstruation (LNYM) affects breast cancer (BC) risk. The aim of this study was to investigate this association in postmenopausal women. <p>

<p>Methods: We used data from the Norwegian Women and Cancer (NOWAC) study, a population-based prospective cohort study. Our study sample included 54 026 postmenopausal women. We estimated hazard ratios and 95% confidence intervals for the association between BC and LNYM categories (<25, 25-29, 30-34, ≥35) with Cox proportional hazards regression. Adjustments were made for age, BMI, physical activity, smoking, and alcohol consumption. <p>

<p>Results: 2 301 women developed BC. We observed a statistically significant increased BC risk in women with LNYM above 35. Using the LNYM category <25 as reference, the hazard ratios for LNYM 25-29, 30-34 and ≥35 were respectively 1.02 (95% CI 0.88-1.19), 1.06 (95% CI 0.93-1.21) and 1.18 (95% CI 1.04-1.35). <p>

<p>Conclusion: By combining reproductive factors such as age at menarche and menopause, parity, breastfeeding, and use of oral contraceptives, we have found that women with LNYM higher than 35 have a significant increased risk of BC compared those with LNYM <25.