In this paper we investigate (1) the mutual causal relationship between first union formation and first childbirth, and (2) the existence of constant common determinants of these two events. It is argued that (unmeasured) common factors reflect differentials among the population in value orientations and in norms about the sequencing of events. We apply event history techniques to retrospective survey data for Spain, allowing for the correlation between unobserved heterogeneity components belonging to each process. Our findings confirm the strong interrelationship between union formation and first birth. After controlling for these common factors, we find that the risk of conception increases immediately at marriage, and it continues to be high during the following four years. Entry into cohabitation produces much smaller increases in the relative risk. The effect of the conception of the first child on union formation is especially strong during pregnancy, but declines sharply after delivery.