Current smoking at menopause rather than duration determines the onset of natural menopause

Background: Smoking has frequently been associated with early menopause. However, studies of this association have been inconclusive with regard to duration and intensity of smoking. A major problem in analyzing the effect of smoking duration on menopausal age is that both exposure and outcome are age-dependent. Methods: We calculated age-specific rates for categories of smoking duration and subsequently computed the rate ratios for occurrence of menopause. We were thus able to model the effect of smoking duration on 2 time scales without assumptions of linearity. We used data from a Dutch population-based cohort comprising 5544 women age 49-70 years who had experienced natural menopause. Results: The rate ratio (RR) for occurrence of menopause was increased in women who smoked in the year of menopause (RR = 1.41; 95% confidence interval = 1.32-1.50). The rate ratio of former smokers was similar to women who never smoked (0.95; 0.89-1.02). Prolonged exposure of smoking did not materially affect the risk of menopause, although the daily number of cigarettes currently smoked could increase the risk. Conclusion: Perimenopausal smoking is apparently more important than smoking history in explaining an earlier age of onset of menopause among women who smoke