Age differences in unions have important implications for a number of demographic and societal outcomes. This study examines patterns of age differences in Swedish marital and childbearing unions during the twentieth century, using administrative register data on all first births (1932-2007) and first marriages (1968-2007). All first births are further analyzed by civil status of the parents, and non-married and married parents are compared. The study discusses the theoretical and methodological importance of distinguishing between age heterogamy (absolute age differences) and age hypergamy (gendered age differences) and examines changes in both measures. Results show that age differences in unions changed only slowly over the twentieth century. Age hypergamy decreased at a slow pace, while age heterogamy showed a u-shaped pattern with increasing heterogamy the last decades. These results are confirmed in quantile analyses. Standardizations are also done to examine the influence of age distribution of first unions. Trends for marital versus childbearing unions are similar overall.