title: Familial factors confound the association between maternal smoking during pregnancy and young adult offspring overweight.
abstract: BACKGROUND
  Smoking during pregnancy has been shown to increase the risks of several adverse birth outcomes. Associations with overweight and/or obesity in the offspring have also been suggested. We aim to investigate whether familial factors confound the association between maternal smoking during pregnancy and overweight in early adulthood in young Swedish males born 1983-88.
  
  
  METHODS
  In a population-based Swedish cohort comprising 124 203 singleton males born to Nordic mothers between 1983 and 1988, we examined the association between maternal smoking during pregnancy and the risk of overweight in the offspring at age ∼18 years. We also investigated the association within siblings, controlling for common genes and shared environment.
  
  
  RESULTS
  In the cohort analyses, the risk of overweight was increased in sons of smoking mothers compared with sons of non-smokers: adjusted odds ratios 1.41, 95% confidence interval (CI) 1.34-1.49, and 1.56, 95% CI 1.46-1.66, for one to nine cigarettes per day, and >10 cigarettes per day, respectively. Stratifying for maternal smoking habits across two subsequent male pregnancies, there was an increased risk of overweight for the second son only if the mother was smoking in both male pregnancies. The effect of smoking during pregnancy on the offspring's body mass index was not present when the association was evaluated within full and half sibling pairs.
  
  
  CONCLUSIONS
  The association between maternal smoking during pregnancy and offspring's risk of overweight appears to be confounded by familial factors.
authors.affiliation: Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Stockholm, Sweden. anastasia.iliadou@ki.se
authors.forename: Anastasia Nyman
authors.initials: AN
authors.lastname: Iliadou
authors.forename: Ilona
authors.initials: I
authors.lastname: Koupil
authors.forename: Eduardo
authors.initials: E
authors.lastname: Villamor
authors.forename: Daniel
authors.initials: D
authors.lastname: Altman
authors.forename: Christina
authors.initials: C
authors.lastname: Hultman
authors.forename: Niklas
authors.initials: N
authors.lastname: Långström
authors.forename: Sven
authors.initials: S
authors.lastname: Cnattingius
mesh_terms: D000328:Adult; D015992:Body Mass Index; D015331:Cohort Studies; D015986:Confounding Factors, Epidemiologic; D005190:Family; D005260:Female; D020022:Genetic Predisposition to Disease; D006801:Humans; D008297:Male; D050177:Overweight / Q000453:epidemiology* / Q000235:genetics; D011247:Pregnancy; D011256:Pregnancy Outcome / Q000453:epidemiology*; D011297:Prenatal Exposure Delayed Effects / Q000453:epidemiology* / Q000235:genetics; D012907:Smoking / Q000009:adverse effects*; D012959:Socioeconomic Factors; D013548:Sweden / Q000453:epidemiology