Couples who have children are increasingly likely to have lived together without being married at some point in their relationship. Some couples begin their unions with cohabitation and marry before first conception, some marry during pregnancy or directly after the first birth, while others remain unmarried 3 years after the first birth. Using union and fertility histories since the 1970s for eleven countries, we examine whether women who have children in unions marry, and if so, at what stage in family formation. We also examine whether women who conceive when cohabiting are more likely to marry or separate. We find that patterns of union formation and childbearing develop along different trajectories across countries. In all countries, however, less than 40 per cent of women remained in cohabitation up to 3 years after the first birth, suggesting that marriage remains the predominant institution for raising children.