title: Is maternal smoking during pregnancy a risk factor for hyperkinetic disorder?--Findings from a sibling design.
abstract: BACKGROUND
  Studies have consistently shown that pregnancy smoking is associated with twice the risk of hyperactivity/inattention problems in the offspring. An association of this magnitude may indicate behavioural difficulties as one of the most important health effects related to smoking during pregnancy. However, social and genetic confounders may fully or partially account for these findings.
  
  
  METHODS
  A cohort including all singletons born in Finland from 1 January 1987 through 31 December 2001 was followed until 1 January 2006 based on linkage of national registers. Data were available for 97% (N = 868,449) of the population. We followed singleton children of smoking and non-smoking mothers until they had an International Classification of Diseases, 10th revision, diagnosis of hyperkinetic disorder (HKD) or to the end of the observation period. We used sibling-matched Cox regression analyses to control for social and genetic confounding.
  
  
  RESULTS
  We found a much smaller association between exposure to maternal smoking during pregnancy and risk of HKD in children using the sibling-matched analysis [hazards ratio (HR) = 1.20, 95% confidence interval (CI) 0.97-1.49] than was observed in the entire cohort (HR 2.01, 95% CI 1.90-2.12).
  
  
  CONCLUSIONS
  Our findings suggest that the strong association found in previous studies may be due to time-stable familial factors, such as environmental and genetic factors. If smoking is a causal factor, the effect is small and less important than what the previous studies indicate.
authors.affiliation: Department of General Practice, School of Public Health, Aarhus University, Denmark. co@alm.au.dk
authors.forename: Carsten
authors.initials: C
authors.lastname: Obel
authors.forename: Jørn
authors.initials: J
authors.lastname: Olsen
authors.forename: Tine Brink
authors.initials: TB
authors.lastname: Henriksen
authors.forename: Alina
authors.initials: A
authors.lastname: Rodriguez
authors.forename: Marjo-Riitta
authors.initials: MR
authors.lastname: Järvelin
authors.forename: Irma
authors.initials: I
authors.lastname: Moilanen
authors.forename: Erik
authors.initials: E
authors.lastname: Parner
authors.forename: Karen Markussen
authors.initials: KM
authors.lastname: Linnet
authors.forename: Anja
authors.initials: A
authors.lastname: Taanila
authors.forename: Hanna
authors.initials: H
authors.lastname: Ebeling
authors.forename: Einar
authors.initials: E
authors.lastname: Heiervang
authors.forename: Mika
authors.initials: M
authors.lastname: Gissler
mesh_terms: D001289:Attention Deficit Disorder with Hyperactivity / Q000209:etiology; D015331:Cohort Studies; D005260:Female; D005333:Fetus / Q000187:drug effects*; D006801:Humans; D006948:Hyperkinesis / Q000209:etiology*; D008297:Male; D011247:Pregnancy; D016016:Proportional Hazards Models; D035781:Siblings; D012907:Smoking / Q000009:adverse effects*