Partnership life courses and completed fertility in Spain

Background: Fertility in Spain has declined and shifted to later ages alongside diversification of family formation, raising questions about how partnership life courses shape completed fertility by late reproductive age. Objective: We aim to assess how key features of partnership trajectories – timing, duration, and sequencing – relate to motherhood and parity by age 41 among women in contemporary Spain, and whether associations differ by education. Methods: We analyze 7,656 women, born 1962–1977, in the Spanish Fertility Survey (2018). A feature-selection approach identifies salient aspects of union histories (e.g., postponement or absence of union, stability, cohabitation, higher-order unions) associated with (1) entry into motherhood and (2) progression to higher-order births by age 41, with results stratified by educational attainment. Results: Delayed or foregone union formation during the reproductive years is strongly associated with lower fertility, particularly a lower likelihood of entering motherhood. Stable marriage is consistently linked to higher probabilities of both becoming a mother and progressing to higher-order births. Cohabitation and post-separation trajectories show more nuanced and generally weaker associations with fertility in these cohorts. Associations between partnership status at later reproductive ages and fertility outcomes are more pronounced among highly educated women. Conclusions: Partnership life courses are central to understanding low and late fertility in Spain, and our results shed light on how traditional and emerging partnership dynamics jointly shape fertility outcomes by late reproductive ages. Contribution: We move beyond a narrow event-based focus to provide an integrated, trajectory-based account of how partnership timing, duration, and sequencing relate to motherhood and parity in a sustained low-fertility setting, documenting educational differences in these associations.