Stoebenau, KirstenMadhavan, SangeethaKim, SeungwanWainaina, CarolDespite repeated calls for improved measures of marriage as a process in sub-Saharan Africa, large-scale surveys continue to rely on static marital status. As a result, there is an incomplete understanding of the effects of marriage on outcomes of interest. We use qualitative and survey data from a longitudinal study of 1,203 young mothers residing in informal settlements of Nairobi, Kenya, to (1) describe the development of an innovative measure of union formalization (UF) defined as the steps through which a union attains social legitimacy; (2) compare UF with a conventional current marital status measure; (3) examine the distribution of UF steps across union history; and (4) examine the sequence and timing of pregnancy and childbearing within the UF process. We find UF steps indicative of both increasing individualization of marriage and the ongoing importance of kin involvement hold meaning. We demonstrate extensive heterogeneity in the sequence and extent of UF steps completed and interrogate the classification of premarital childbearing using sequence analysis. We argue that measuring UF is feasible and necessary for the next generation of family demography in Africa; UF measures facilitate understanding the linkages among family dynamics, health, and social stratification within the context of ongoing socioeconomic change.en-USArticle