Biological Age Meets Couple Duration: Untangling Timing of First Births in the Context of Fertility Decline

Kelsey Q Wright , Max Planck Institute for Demographic Research
Angela Carollo, Max Planck Institute for Demographic Research
Mikko Myrskylä, Max Planck Institute for Demographic Research

In this analysis, we examine how biological age and couple duration jointly account for the timing and achievement of first births using a novel two-time-scale event history approach. Using Finnish population register data (1987–2018) and German pairfam panel data (2008– 2022), we follow childless individuals aged 18–49 from the start of cohabiting or marital unions until first birth or censoring, and stratify our results by gender and level of education. In Finland, first-birth hazards peak earlier and more diffusely, whereas in Germany, transitions to parenthood are more tightly linked to biological aging—particularly among women entering unions after age 35. Men’s trajectories exhibit greater heterogeneity, with late-life peaks suggesting compensatory childbearing. We examine educational gradients in the Finnish context and find pronounced differences: Finnish women with tertiary education tend to delay childbearing within longer cohabitations, while those with primary education transition to first births early, often within shorter cohabitation durations. Our findings show that fertility postponement reflects both delayed union formation and extended childless cohabitation, varying by socioeconomic status and country context. Modeling biological and relational time simultaneously advances understanding of the mechanisms driving fertility decline and reveals how reproductive timing constraints are jointly structured by partnership and age.

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 Presented in Session 1. Flash Session Fertility and Health