A strong relationship has been established between a mother's age at first birth and her daughter's age at first birth. Using a theoretical framework derived from the literature and a long-term intergenerational panel study of White mother–child pairs, this article (1) establishes that the intergenerational transmission of first birth timing is also strong among sons, (2) provides a theoretical model and empirical evidence that the intergenerational transmission of first birth timing is limited to premaritally conceived first births, and (3) shows that the social circumstances in which children of young mothers are raised fully account for the intergenerational transmission of first birth timing. Among young women, increased rates of premarital childbearing are explained by their grandfather's occupation, mother's religious affiliation, mother's contraceptive use, mother's total number of children, family income and financial assets, mother's education, and mother's divorce followed by remarriage. Among young men, increased rates of premarital childbearing are explained mainly by their mother's education. The results point to the need for further research on why these characteristics of families increase children's premarital first birth rates.