Objective: This article examines relationships between reproductive onset and lifetime history of smoking, regular smoking, and nicotine dependence, and cannabis and other illicit drug use. Method: Data were drawn from a young adult cohort of 3,386 female and 2,751 male Australian twins born between 1964 and 1971. Survival analyses were conducted using Cox proportional hazards regression models predicting age at first childbirth from history of substance use or disorder separately by substance class. Other substance use or disorder, including alcohol dependence, as well as sociodemographic characteristics, history of psychopathology, and family and childhood risks, were included as control variables in adjusted models. Results: Regular smoking and nicotine dependence were associated with earlier reproduction, with pronounced effects for women. For women, use of cannabis was associated with early reproduction before age 20, and with delayed reproduction among women who have not reproduced by age 20 or 25. Adjustment for control variables only partially explained these associations. Conclusions: Consistent with research linking adolescent use with sexual risk taking predictive of early childbearing, regular smokers and nicotine-dependent individuals show earlier reproductive onset. In contrast, delays in childbearing associated with use of cannabis are consistent with impairments in reproductive ability and/or opportunities for reproduction. Continued research on risks both upstream and downstream of substance-use initiation and onset of substance-use disorder is needed for causal mechanisms to be fully understood.