Determining the ontogeny of the human fetal adaptive immune system has important implications for defining the infectious risk of prematurely born infants. However, due to technical limitations, little is known about the full diversity of the fetal immune repertoire. In this study, the authors used next-generation sequencing technology to analyse B cell and T cell lymphopoiesis in blood samples from human fetuses from 12 to 26 weeks of gestation. The data confirm that B cell development precedes T cell development and show that there is a progressive increase in both B cell and T cell receptor repertoire diversity during human fetal development. Furthermore, somatic hypermutation and class-switch recombination occur during fetal B cell development, even at a time when there is limited T cell lymphopoiesis. These data help to define the level of immune competency in developing fetuses and may be used as a reference for future studies of fetal immunity.