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 1950s 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. STUDY FUNDING/COMPETING INTEREST(S) This work was funded by the South-Eastern Norway Regional Health Authority [grant number 2016112 to M.S.G.] and by the Norwegian Cancer Society [grant number 6863294-2015 to E.K.B.]. The authors declare no conflicts of interest.

STUDY QUESTION Does age at natural menopause increase with increasing of number of childbirths? SUMMARY ANSWER Age at menopause increased with increasing number of childbirths up to three childbirths; however, we found no further increase in age at menopause beyond three childbirths. WHAT IS KNOWN ALREADY Pregnancies interrupt ovulation, and a high number of pregnancies have therefore been assumed to delay menopause. Previous studies have had insufficient statistical power to study women with a high number of childbirths. Thus, the shape of the association of number of childbirths with age at menopause remains unknown. STUDY DESIGN, SIZE, DURATION A retrospective population study of 310 147 women in Norway who were 50–69 years old at data collection. PARTICIPANTS/MATERIALS, SETTING, METHODS The data were obtained by two self-administered questionnaires completed by women attending BreastScreen Norway, a population-based screening program for breast cancer. The associations of number of childbirths with age at menopause were estimated as hazard ratios by applying Cox proportional hazard models, adjusting for the woman’s year of birth, cigarette smoking, educational level, country of birth, oral contraceptive use and body mass index. MAIN RESULTS AND THE ROLE OF CHANCE Women with three childbirths had the highest mean age at menopause (51.36 years; 95% CI: 51.33–51.40 years), and women with no childbirths had the lowest (50.55 years; 95% CI: 50.48–50.62 years). Thus, women with no childbirths had higher hazard ratio of reaching menopause compared to women with three childbirths (reference group) (adjusted hazard ratio, 1.24; 95% CI: 1.22–1.27). Beyond three childbirths, we estimated no further increase in age at menopause. These findings were confirmed in sub-analyses among (i) women who had never used hormonal intrauterine device and/or systemic menopausal hormonal therapy; (ii) women who were born before 1950 and (iii) women who were born in 1950 or after. LIMITATIONS, REASONS FOR CAUTION Information about age at menopause was based on self-reports. WIDER IMPLICATIONS OF THE FINDINGS If pregnancies truly delay menopause, one would expect that women with the highest number of childbirths had the highest age at menopause. Our results question the assumption that interrupted ovulation during pregnancy delays menopause. STUDY FUNDING/COMPETING INTEREST(S) This work was supported by the South-Eastern Norway Regional Health Authority [2016112 to M.S.G.] and by the Norwegian Cancer Society [6863294-2015 to E.K.B.]. The authors declare no conflicts of interest. TRIAL REGISTRATION NUMBER N/A.

This study showed that more than 50% of the women who reached menopause before the age of 45 years had had a successful pregnancy within 10 years prior to natural menopause. Among women with menopause in their mid-fifties, less than 1% had had a successful pregnancy within the 10 years before natural menopause. ABSTRACTResearch question: Does a successful spontaneous pregnancy in the years close to natural menopause depend on age at menopause?Design: This was a retrospective population-based study of 4157 parous postmenopausal women in Norway, born during the years 1925-1940. Data were obtained by two self-administered questionnaires in the HUNT2 Survey (1995)(1996)(1997). We calculated the proportions of women who gave birth within 5 years and within 10 years prior to menopause both among all women, and according to categories of age at menopause. Results:Overall, 2.7% (114/4157) of all women gave birth within 5 years, and 11.7% (487/4157) gave birth within 10 years, prior to menopause. Among women with menopause before the age of 45 years, 23.5% (81/344) gave birth within 5 years, and 55.5% (191/344) gave birth within 10 years, before menopause. Among the women with menopause at the age of 55 years or older, no women (0/474) gave birth within 5 years, and 0.2% (1/474) gave birth within 10 years, prior to menopause. Conclusions:More than half of the women with menopause before the age of 45 years gave birth within the 10 years before natural menopause, whereas virtually no women with menopause at the age of 55 years or older did. Thus, the length of the sterile interval before natural menopause may vary by age at menopause.