Introduction Couples sometimes have fewer children than they intend to, resulting in a gap between intended family size and completed fertility. Education and union patterns are considered important factors behind postponement of fertility, which in turn is a key component of the fertility gap; because physiological constraints on fertility intensify at older reproductive ages. We aim to measure how much education and union patterns contributed to the fertility gap of Dutch women born during 1974-1984 (mean of 0.23, Table 2). Research question How much did union formation and - dissolution as well as educational attainment contribute to the gap between intended family size and completed fertility for Dutch women born between 1974 and 1984? Theory We assume that most pregnancies are a conscious decision by a couple. However, significant shares of pregnancies are unplanned and unintended. Some couples who want to get pregnant are also unable to, due to low fecundity and miscarriage. By including contraceptive behaviour and physiological constraints on fertility we model both perceived and actual control over fertility. Figure 1: Conceptual framework based on literature review Data Methods We simulate the reproductive life courses of individual women (couples). We predict the fertility of the cohort by generating cumulative distribution functions based on cohort distributions and parameters derived from the data, from which we make random draws. Simulation models The simulation model is split in to two separate parts, the union formation and reproductive processes. The models iterate monthly from age 15 to 55 (or sterility) to match the reproductive window and the menstrual cycle. No conceptions occur during enrolment in education or outside of cohabitation. We simulate 100,000 women. Figure 2: Union formation simulation model (woman stays in her current state if the transition conditions are not met) Figure 3: Reproductive process simulation model (IUM = Intrauterine mortality) Results Education barely contributes to the fertility gap, because later and less partnering among highly educated women is compensated for by more stable unions. Moderate changes in marriage, separation, and re-partnering all have small contributions to the gap. Divorce essentially has no contribution (70% marry, event occurs late in life course). The contribution of postponement of first cohabitation increases with duration due to physiological constraints on fertility at older ages. A reduction in the share of women who ever partner contributes substantially to the gap. Figure 4: Distribution (%) of women by intended family size and completed fertility. Most women realised their fertility intentions (diagonal line), underachievement (below line) was much more common than overachievement (above line). The fertility gap increased somewhat with education. Table 1: Contributions to the fertility gap (mean 0.33) Parameters Adjustment Contribution to fertility gap Share of highly educated Increase by 10%-points 0.013 Share of highly educated 25-34 year-olds in 2022 0.010 Share who marry Decrease by 5%-points 0.023 Share who separate Increase by 5%-points 0.019 Share who divorce Increase by 5%-points 0.003 Share who re-partner Decrease by 5%-points 0.027 Age at first cohabitation Increase mean by 1 year 0.029 Age at first cohabitation Increase mean by 3 years 0.061 Age at first cohabitation Increase mean by 5 years 0.181 Share that ever cohabit Decrease by 5%-points 0.092 Table 2: Simulation results versus reference data Indicator Demographic data Simulation results (1974- 1984 cohort) Mean age at first marriage (CBS) 30.10 29.22 Mean age at first cohabitation 24.50 24.30 Mean age at first separation (no previous divorce) 28.50 28.15 Mean age at first divorce 37.60 38.07 Mean age at first repartnering NA 33.32 Mean number of partners (1954- 1964) 1.20 1.43 Percent ever cohabited 95.00 95.05 Percent ever married 70.00 72.20 Percent cohabited & never separated