1 Environmental Medicine, Department of Public Health, Det Sundhedsvidenskabelige Fakultet, SDU2 Epidemiology, Biostatistics and Biodemography, Department of Public Health, Det Sundhedsvidenskabelige Fakultet, SDU3 Bispebjerg-Frederiksberg University Hospital4 Institute of Preventive Medicine, Bispebjerg and Frederiksberg Hospitals, The Capital Region, Denmark; Novo Nordisk Foundation Centre for Basic Metabolic Research, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark.5 Epidemiology, Biostatistics and Biodemography, Department of Public Health, Det Sundhedsvidenskabelige Fakultet, SDU6 Environmental Medicine, Department of Public Health, Det Sundhedsvidenskabelige Fakultet, SDU
OBJECTIVE: To investigate the association between exposure to mothers smoking during prenatal and early postnatal life and risk of overweight at age 7 years, while taking birth weight into account. METHODS: From the Danish National Birth Cohort a total of 32,747 families were identified with available information on maternal smoking status in child's pre- and postnatal life and child's birth weight, and weight and height at age 7 years. Outcome was overweight according to the International Obesity Task Force gender and age specific body mass index. Smoking exposure was categorized into four groups: no exposure (n = 25,076); exposure only during pregnancy (n = 3,343); exposure only postnatally (n = 140); and exposure during pregnancy and postnatally (n = 4,188). Risk of overweight according to smoking status as well as dose-response relationships were estimated by crude and adjusted odds ratios using logistic regression models. RESULTS: Exposure to smoking only during pregnancy, or both during pregnancy and postnatally were both significantly associated with overweight at 7 years of age (OR: 1.31, 95% CI: 1.15-1.48, and OR: 1.76, 95% CI: 1.58-1.97, respectively). Analyses excluding children with low birth weight (<2,500 gram) revealed similar results. A significant prenatal dose-response relationship was found. Per one additional cigarette smoked per day an increase in risk of overweight was observed (OR: 1.02, 95% CI: 1.01-1.03). When adjusting for quantity of smoking during pregnancy, prolonged exposure after birth further increased the risk of later overweight in the children (OR 1.28, 95% CI:1.09-1.50) compared with exposure only in the prenatal period. CONCLUSIONS: Mother's perinatal smoking increased child's OR of overweight at age 7 years irrespective of birth weight, and with higher OR if exposed both during pregnancy and in early postnatal life. Clear dose-response relationships were observed, which emphasizes the need for prevention of any tobacco exposure of infants.