title: Birth weight as an independent predictor of ADHD symptoms: a within-twin pair analysis.
abstract: BACKGROUND
  Studies have found an association between low birth weight and ADHD, but the nature of this relation is unclear. First, it is uncertain whether birth weight is associated with both of the ADHD dimensions, inattentiveness and hyperactivity-impulsivity. Second, it remains uncertain whether the association between birth weight and ADHD symptom severity is confounded by familial factors.
  
  
  METHODS
  Parents of all Swedish 9- and 12-year-old twins born between 1992 and 2000 were interviewed for DSM-IV inattentive and hyperactive-impulsive ADHD symptoms by the Autism - Tics, AD/HD and other Comorbidities (A-TAC) inventory (N = 21,775 twins). Birth weight was collected prospectively through the Medical Birth Registry. We used a within-twin pair design to control for genetic and shared environmental factors.
  
  
  RESULTS
  Reduced birth weight was significantly associated with a mean increase in total ADHD (β = -.42; 95% CI: -.53, -.30), inattentive (β = -.26; 95% CI: -.33, -.19), and hyperactive-impulsive (β = -.16; 95% CI: -.22, -.10) symptom severity. These results imply that a change of one kilogram of birth weight corresponded to parents rating their child nearly one unit higher (going from "no" to "yes, to some extent" on a given symptom) on the total ADHD scale. These associations remained within pairs of MZ and DZ twins, and were also present when restricting the analyses to full term births.
  
  
  CONCLUSIONS
  There is an independent association between low birth weight and all forms of ADHD symptoms, even after controlling for all environmental and genetic confounds shared within twin pairs. These results indicate that fetal growth restriction (as reflected in birth weight differences within twin pairs) and/or the environmental factors which influence it is in the casual pathway leading to ADHD.
authors.affiliation: Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Stockholm, Sweden.
authors.forename: Erik
authors.initials: E
authors.lastname: Pettersson
authors.forename: Arvid
authors.initials: A
authors.lastname: Sjölander
authors.forename: Catarina
authors.initials: C
authors.lastname: Almqvist
authors.forename: Henrik
authors.initials: H
authors.lastname: Anckarsäter
authors.forename: Brian M
authors.initials: BM
authors.lastname: D'Onofrio
authors.forename: Paul
authors.initials: P
authors.lastname: Lichtenstein
authors.forename: Henrik
authors.initials: H
authors.lastname: Larsson
mesh_terms: D001289:Attention Deficit Disorder with Hyperactivity / Q000175:diagnosis / Q000453:epidemiology / Q000209:etiology*; D001724:Birth Weight / Q000502:physiology*; D002648:Child; D004200:Diseases in Twins / Q000175:diagnosis / Q000453:epidemiology / Q000209:etiology*; D005260:Female; D006801:Humans; D008297:Male; D011379:Prognosis; D012042:Registries; D013548:Sweden / Q000453:epidemiology
keywords: ADHD; DSM; behavioral genetics; birth weight; environmental influences