Research on migrant fertility has often found differences between the childbearing of migrants and natives. These ‘differentials’ are important because they demonstrate how migrants contribute towards population change. They can be also used to investigate how living in a new destination affects the fertility of immigrants and their descendants. This is especially true when demographers study how differentials change over time as a process of convergence. Unfortunately, the literature on migrant fertility differentials suffers from a number of limitations. Firstly, existing definitions of migrant fertility convergence are ambiguous. It is unclear what the concept means, and how it should be tested. Secondly, researchers have limited knowledge about variation in differentials over the life course, in particular for women who have completed childbearing. Thirdly, there is a lack of empirical research that examines why differentials exist, and whether they can be explained by exposure to cultural norms. This thesis responds to these issues with four papers, one that critically evaluates convergence, and three that analyse migrant fertility in the UK. The results show evidence of generational convergence for some descendants of immigrants, notably those with Irish and Jamaican ancestry, but evidence against convergence for the descendants of immigrants from Pakistan and Bangladesh. These results are partly explained by childhood socialisation and culturally entrenched fertility norms, such that differentials are lower for child migrants who grow up in areas where they are more likely to be exposed to native cultural norms. Overall, the results show that differentials vary considerably over the life course, and follow very different patterns for different migrant groups. The findings suggest that researchers must be careful when trying to make generalisations about migrant fertility behaviour. They also highlight the immigrants, descendants, life course stages, and explanations of migrant fertility that may be most fruitfully studied by future research.