In this paper, we investigate through microsimulation the contribution of union status at first birth (cohabitation versus marriage) to parental separation. We estimate parameters predicting union and birth transitions from the intersection of a parent’s prior childbearing and union history, as observed in nationally representative surveys in Italy, Great Britain and Scandinavia, represented by Norway and Sweden. Those parameters are used to generate hypothetical populations with the predicted distribution of union/birth histories for each country and cohort. We then use the populations to decompose changes in parental separation across birth cohorts, identifying how much of the change can be attributed to shifts in union status at birth. We further investigate the degree to which shifts in age at first birth may account for or offset the contribution of cohabitation to increases in parental separation. We compare results for national contexts with quite different levels and shifts over time in parental cohabitation and separation.