Although an expanding literature has focused on union formation between migrants and natives, little research has so far examined how fertility dynamics may differ between couples in an endogamous and ethnic exogamous union. The aim of this paper is to analyze both married and cohabiting couples of diverse ethnic origins in Australia and their decision of entering into parenthood by considering the role of their fertility intentions and other characteristics in terms of dissimilarities between partners. For this study unique data come from the Household, Income and Labour Dynamics in Australia survey (HILDA) that is one of the few datasets with longitudinal couple-level information. Our results support the exogamy hypothesis: first-generation exogamous couples where the male partner is the migrant face a lower risk of having a first child after controlling for couple’s socio-economic characteristics. We also find support for the adaptation hypothesis: second-generation exogamous unions have similar risk than Australian native unions of having a first child. Finally, results show that the effect of exogamy contributes to decrease first-child risk rate if the immigrant partner comes from a non-English speaking country and in absence of religious homogamy between partners.