In a situation of low fertility like that operating nowadays, studying the determinants of reproductive behaviour is very important. In a context such as Western industrialized one, in which birth control induces a feature of consciousness into fertility conduct, reproductive intentions have a fundamental role. This paper examines the mechanism which leads to the formation of reproductive behaviour starting from intentions, with reference to a large city in Italy, where fertility is very low, Milan. In addition, the hypothesis that women have innate and unobservable propensities toward family formation is verified through a simultaneous equations model. It allows to control for the potential endogeneity of reproductive intentions in determining subsequent behaviour. Results partially confirm this hypothesis: no significant correlation is found between unobservable components influencing intentions and behaviour. The relevance of reproductive intentions for subsequent fertility behaviour is, however, pointed out.