Body composition, smoking, and spontaneous dizygotic twinning.

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.