This paper examines the childbearing behaviour of Polish migrant women and their descendants in Sweden. Also considering stayers in the country of origin, we rely on a country-of-origin and country-of-destination approach in a careful examination regarding the relevance of three hypotheses on migrant fertility: the socialisation, selection, and adaptation hypotheses. We analyse the transitions to first and second births based on a piecewise exponential model, using Swedish register data and the Polish Generations and Gender Survey (GGS) first wave. The results support the socialisation hypothesis, as the Polish stayers and the first-generation Polish migrants have their first child at younger ages and are less likely to remain childless than the other groups but are also more likely to not proceed to a second child, unlike the Swedish natives and the second generation. We find partial support for the selection hypothesis. Descriptively, we observe signs of selection into migration based on education, cohort, and marital status. Additionally, our study shows that the impact of marriage varies between stayers and migrants, in the first-birth transition, suggesting selection into migration when it comes to unobserved characteristics as well. The adaptation hypothesis is also supported, as the fertility behaviour of the second generation more closely resembles that of the Swedish natives than that of the first generation and differs more from that of the Polish stayers in terms of both quantum and timing of the first and second births.