Childlessness is the outcome of a complex process, which involves social as well as physiological factors, both of which likely interact in producing reproductive behaviors and outcomes. Demographic studies on childlessness have typically focused on the social underpinnings of fertility behaviors. However, recent research indicates that health during young adulthood, in particular BMI, is a significant and robust predictor of population-level variation in lifetime fertility. Obese men and women and underweight men tend to remain childless significantly more often. Little, however, is known on the mechanisms which underlie this association. Using data from the NLSY79, we test a variety of pathways via which BMI may be linked to childlessness. We consider three health pathways (general health, reproductive health, mental health), and the pathway of union formation chances, timing, and duration. We also examine sex differences in the relevance of these potential pathways underlying the BMI-childlessness nexus.