Temporal trends in age at menarche and age at menopause: a population study of 312 656 women in Norway.

STUDY QUESTION: Have mean age at menarche or mean age at natural menopause changed from the 1939 birth cohort to the 1964 birth cohort? SUMMARY ANSWER: We estimated a minor decrease in mean age at menarche and an increase by nearly 3 years in mean age at natural menopause. WHAT IS KNOWN ALREADY: In the Western world, age at menarche decreased across birth cohorts from the early 1800s until the 1950s. Whether mean age at menarche has continued to decrease in birth cohorts after the I 950s remains uncertain. It is also uncertain whether mean age at natural menopause has changed across birth cohorts. STUDY DESIGN, SIZE, DURATION: We performed a retrospective population study of 312 656 women who were born in Norway during the years 1936-1964. PARTICIPANTS/MATERIALS, SETTING, METHODS: The data were obtained by two self-administered questionnaires from women who participated in the Norwegian breast cancer screening program (BreastScreen Norway) during the years 2006-2014. We used flexible parametric survival models with restricted cubic splines to estimate mean age at menarche, mean age at menopause and mean number of years between menarche and menopause according to the women's year of birth. The women who were still having menstrual periods contributed with follow-up time until the time of data collection, and the women who had reported surgical removal of the uterus and/or both ovaries prior to natural menopause contributed with follow-up time until the time of surgery. MAIN RESULTS AND THE ROLE OF CHANCE: The mean age at menarche was 13.42 years (95% CI: 13.40-13.44 years) among women born during 1936-1939, and it was 13.24 years (95% CI: 13.22-13.25 years) among women born during 1960-1964. The mean age at natural menopause increased from 50.31 years (95% CI: 50.25-50.37 years) among women born during 1936-1939 to 52.73 years (95% CI: 52.64-52.82 years) among women born during 1960-1964. The mean number of years between menarche and menopause increased from 36.83 years (95% CI: 36.77-36.89 years) to 40.22 years (95% CI: 40.11-40.34 years). LIMITATIONS, REASONS FOR CAUTION: Information about age at menarche and age at menopause was based on self-reports. WIDER IMPLICATIONS OF THE FINDINGS: Late menopause is associated with increased risk of breast cancer but also with increased life expectancy. Thus, higher mean age at menopause may partly explain the increase in breast cancer incidence after menopause and the increase in life expectancy in recent time. Also, a longer interval between menarche and menopause could suggest that the number of years of female fecundity has increased.