We investigate the effect of parenthood on whether non-marital unions led to marriage or parting for two cohorts of British women when they were aged between 16 and 29. We compare the effect of conceptions leading to births and the presence and characteristics of children on the odds that a cohabitation was dissolved, or that it was converted to marriage, for women born in 1958 and 1970. A multilevel, multiprocess, competing-risks model allows for multiple cohabitation per woman and endogeneity of fertility status. We find that cohabiting couples' response to impending parenthood and the presence of children changed over time. In particular, the proportion of cohabiting couples who married before a birth decreased and, in the 1970 cohort only, the risk of dissolution declined during pregnancy. There is also evidence that the presence of a child cemented a cohabiting union for women from the 1970, but not the earlier, cohort.