Finland has been a demographic forerunner in terms of the adoption of new family forms as well as family friendly social policies. Childlessness has nevertheless grown and, is high compared to many other countries. A large and increasing share of all persons who live in unions are childless. Using population-register data for the cohorts born 1952-1966, we study persons who were partnered at age 45 (n = 44,321). The aim is to analyse how marital status and union duration, together with socioeconomic characteristics of the ego and the partner, relate to the probability of being childless at 45. Logistic regression models are estimated separately for women and men, using a 10-year retrospective window. We find for both men and women, the strongest marker for being a parent by age 45 is whether they are married and had lived with the same partner for at least ten years. Shorter union durations and cohabitations are associated with a several-fold increase in the likelihood of being childless. Socioeconomic characteristics are relevant as well, but notably less important in terms of estimated effect sizes. Future research on this topic should tentatively be concerned with how childlessness relates to transitions into, from, and across unions.