Cohort Trends in the Association between Age at Menarche and Fertility Timing across 40 Years in Norway

Kathryn Beck, Norwegian Institute of Public Health
Øystein Kravdal, Norwegian Institute of Public health
Maria Magnus, Norwegian Institute of Public Health
Jonathan Wörn
Martin Flatø, Norwegian Institute of Public Health and University of Oslo

Age at menarche is a key marker of female reproductive development, yet its long-term implications for fertility remains underexplored beyond adolescence. Using Norwegian health survey data linked to administrative registers, we examine how age at menarche relates to fertility trajectories across the reproductive life course across four decades of birth cohorts (1938–1978). Our analytical sample includes 62,645 women categorized into early (7–11 years), average (12–14 years), and late (15+ years) menarche groups. Piecewise constant hazard models and logistic regressions estimate cohort-specific and menarcheal group differences in the timing and likelihood of first, second, and third births. Results show that early menarche was historically associated with a faster transition to first birth during adolescence, but this effect disappears among the younger cohorts. Conversely, late menarche consistently predicts slower entry into motherhood, particularly in early childbearing years, with this association strengthening in recent cohorts. We also find emerging patterns of inequality in fertility. In the cohorts born between 1958–1978, both early and late menarche groups exhibit higher childlessness at age 45 and lower odds of second births compared to the average menarche group. These findings suggest that pubertal timing influences not only adolescent fertility but also lifelong reproductive patterns, with implications for understanding demographic shifts and inequalities in fertility. By capturing fertility trajectories across the entire reproductive life course and multiple cohorts, this study provides new evidence on how early and late menarche shape long-term fertility outcomes and may have implications for growing inequalities in childlessness.

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 Presented in Session 1. Flash Session Fertility and Health