Premarital fertility, defined as giving birth before a woman’s first marriage, is prevalent in sub-Saharan Africa. According to DHS data from surveys in 25 countries, an average of one in five women has a birth before marriage. The prevalence of premarital fertility varies markedly, ranging from 2.4 to 60.2 percent in the countries covered. The variation is even greater by ethnic group, ranging from 0.1 to 76.2 percent. The level of premarital fertility is determined by a number of factors including age at first marriage, age at first intercourse, contraceptive use, and various cultural factors; and all of these factors vary by ethnicity. An analysis of 263 ethnic groups shows that age at first marriage is by far the most important factor. Variables linked to modernization—urban residence, modern education, wealth, and Christianity—have positive correlations with premarital fertility, whereas living in a polygynous union and being Muslim have negative correlations. Even taking into account demographic and socioeconomic factors, variations in the level of premarital fertility are substantial, indicating the strong effect of cultural factors. The demographic evidence presented in this analysis is consistent with the ethnographic record, which distinguishes between societies that are culturally conservative and societies that are more open and permissive. The Appendix presents detailed information on premarital fertility by ethnicity for the countries in the study.