OBJECTIVE: To examine differences in body composition and smoking between mothers of spontaneous monozygotic and dizygotic twins, while taking into account maternal age, gravidity, and educational attainment. DESIGN: Retrospective cohort study. SETTING: The Netherlands Twin Register. PATIENT(S): Mothers of twins (n = 19,357) registered with the Netherlands Twin Register. Data were selected from mothers of spontaneous monozygotic twins (MZ; n = 5663) and mothers of spontaneous dizygotic twins (DZ; n = 8515). INTERVENTION(S): None. MAIN OUTCOME MEASURE(S): The odds of having spontaneous DZ twins versus spontaneous MZ twins as a function of height, body mass index (BMI), and smoking before pregnancy, after accounting for age, gravidity, and educational attainment. RESULT(S): Compared with spontaneous MZ twinning, spontaneous DZ twinning is significantly associated with increasing height (odds ratio, 1.6; 95% confidence interval [CI], 1.5-1.8 for the tallest versus the shortest height quartile), an increased BMI (odds ratio, 1.3; 95% CI, 1.1-1.4 for overweight vs. normal weight), and smoking before the twin pregnancy (odds ratio, 1.4; 95% CI, 1.3-1.5 for smoker vs. nonsmoker). Maternal age and gravidity, but not educational attainment, had to be included in the model. CONCLUSION(S): Spontaneous dizygotic twinning is associated with body composition and smoking.