Age at first union is increasing throughout much of sub-Saharan Africa at the same time that not all couples are waiting for marriage before their first sexual intercourse. We assessed the effect of a premarital first birth on entrance into a first union in an urban area in East Africa—Moshi, Tanzania. The data come from the Moshi Infertility Survey of 2002–2003. Women who spent less than a year in single motherhood were significantly more likely than childless women to enter into a first union, although the magnitude of this relationship was weaker for more recent cohorts. Women who had been single mothers for 5 or more years (about two-thirds of women with a premarital birth) were significantly less likely than women without children to enter into a first union.