Table II. Temporal trends in age at menarche, age at menopause and number of years between menarche and menopause among 312 656 women in the BreastScreen Norway, born during the years 1936–1964.

		Age at menarche (years)				Age at menopause (years)				Interval a (years)
	No. women	Mean	95% CI	HR	95% CI	Mean	95% CI	HR	95% CI	Mean	95% CI	HR	95% CI
Birth cohort
1936–1939	17 409	13.42	13.40–13.44	Reference		50.31	50.25–50.37	Reference		36.83	36.77–36.89	Reference
1940–1944	45 831	13.32	13.30–13.33	1.08	1.06–1.10	50.80	50.76–50.83	0.88	0.86–0.90	37.44	37.39–37.48	0.86	0.85–0.88
1945–1949	63 716	13.29	13.28–13.30	1.10	1.08–1.12	50.70	50.66–50.73	0.90	0.89–0.92	37.35	37.31–37.38	0.88	0.86–0.90
1950–1954	70 507	13.21	13.20–13.22	1.17	1.15–1.19	50.93	50.90–50.97	0.85	0.83–0.86	37.80	37.76–37.84	0.79	0.78–0.81
1955–1959	72 789	13.18	13.17–13.19	1.20	1.18–1.22	51.86	51.80–51.91	0.68	0.67–0.70	39.18	39.11–39.25	0.59	0.58–0.60
1960–1964	42 404	13.24	13.22–13.25	1.15	1.13–1.17	52.73	52.64–52.82	0.57	0.55–0.58	40.22	40.11–40.34	0.49	0.48–0.50

The associations of birth cohort with age at menarche, age at menopause and number of years between menarche and menopause are estimated as crude HRs with 95% CI by applying flexible parametric models.

^{a a}Interval = number of years between menarche and menopause.

HR = hazard ratio.