In this paper, we examine the determinants of fertility timing of unmarried and married mothers using a rich new birth cohort study, the Fragile Families and Child Wellbeing Study, drawn from 20 medium and large U.S. cities. We find considerable variation in the time to next birth among comparable mothers who live in different cities. Some of this variation is explained by variation in labor markets, housing costs and availability, and welfare policies. City variation is particularly important for unmarried women who already have two or more children, whose fertility is more sensitive to these contextual variables than is the fertility of married women, or unmarried women with just one child.