The study describes patterns of premarital fertility in 25 countries from sub-Saharan Africa. Premarital fertility is defined as giving birth before a woman’s first marriage, whether formal or informal. Data were taken from 43 DHS and related surveys. On average in the selected countries, about one woman out of five experiences a premarital birth. Prevalence of premarital fertility varies markedly by country (from 2.4% to 60.2%), and even more by ethnic group (from 0.1% to 76.2%). Premarital fertility is the result of a complex process, influenced by age at first marriage, age at first intercourse, contraceptive use and cultural factors, which all vary by ethnicity. An analysis of 263 ethnic groups shows that age at first marriage is by far the most important of these factors. In addition, factors linked to modernization and religion also have an influence on the prevalence of premarital fertility: urbanization, modern education, wealth and being Christian have a positive correlation with premarital fertility, whereas being Muslim or in a polygamous union have negative correlations. However, even taking into account demographic and socioeconomic factors, the variance of premarital fertility remained high, showing the effect of cultural factors. The demographic evidence presented in this analysis appeared consistent with the ethnographic record, which distinguishes between repressive and permissive societies. The Appendix presents the details of premarital fertility by ethnicity for each country considered.