This paper explores how attitudes towards childbearing change after cohabitation and marriage. Entering in a union may affect attitudes towards childbearing, but at the same time individuals who are more oriented towards childbearing may be more determined to form unions. In order to disentangle the interplay between union formation and subsequent changes in attitudes towards childbearing we implement propensity score matching, which is applied to a panel data coming from a survey in Bulgaria to identify the effect of union formation on attitudes related to childbearing. This method controls for observable confounders which may affect both the probability of entering into a union and childbearing preferences, reducing selection bias. We find a positive and significant causal relationship of entering into union on attitudes towards parenthood among men, whereas the effect is weaker and often uncertain for women.