title: Birth weight and attention-deficit/hyperactivity symptoms in childhood and early adolescence: a prospective Swedish twin study.
abstract: OBJECTIVE
  To determine whether low birth weight increases the risk of attention-deficit/hyperactivity disorder (ADHD) in childhood and early adolescence.
  
  
  METHODS
  In a population-based sample of 1,480 twin pairs born in the period 1985-1986 ascertained from the Swedish Twin Registry, birth weight was collected prospectively through the Medical Birth Registry. ADHD symptoms were measured with a 14-item checklist covering DSM-III-R criteria (parental rating) at age 8 to 9 years and 13 to 14 years. We used both a dichotomous approach for birth weight (>400 g or 15% weight difference) and ADHD (eight or more symptoms) and continuous measures to investigate between- and within-twin pair effects.
  
  
  RESULTS
  Our results showed that low birth weight was a risk factor for symptoms of ADHD and the associations did not diminish when we controlled for genetic influence. The lighter twin in birth weight-discordant pairs had on average 13% higher ADHD symptom score at age 8 to 9 years (p = .006) and 12% higher ADHD score at age 13 to 14 years (p = .018) compared with the heavier twin. The genetic correlations suggest modest or no genetic overlap between birth weight and ADHD.
  
  
  CONCLUSIONS
  The hypothesis that low birth weight is associated with the development of ADHD symptoms was supported in this prospective twin study. Fetal growth restriction seems to represent a modest but fairly consistent environmental influence on the development of ADHD symptoms.
authors.affiliation: Dr. Hultman is with the Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Stockholm, and the Department of Neuroscience Psychiatry, Ulleråker, University of Uppsala, Sweden; Ms. Torrång, Ms. Tuvblad, and Drs. Cnattingius and Lichtenstein are with the Department of Medical Epidemiology and Biostatistics, Karolinska Institutet; and Dr. Larsson is with the Department of Child and Adolescent Psychiatry, Karolinska University Hospital, Stockholm, Sweden. Electronic address: Christina.Hultman@ki.se.
authors.forename: Christina M
authors.initials: CM
authors.lastname: Hultman
authors.affiliation: Dr. Hultman is with the Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Stockholm, and the Department of Neuroscience Psychiatry, Ulleråker, University of Uppsala, Sweden; Ms. Torrång, Ms. Tuvblad, and Drs. Cnattingius and Lichtenstein are with the Department of Medical Epidemiology and Biostatistics, Karolinska Institutet; and Dr. Larsson is with the Department of Child and Adolescent Psychiatry, Karolinska University Hospital, Stockholm, Sweden.
authors.forename: Anna
authors.initials: A
authors.lastname: Torrång
authors.affiliation: Dr. Hultman is with the Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Stockholm, and the Department of Neuroscience Psychiatry, Ulleråker, University of Uppsala, Sweden; Ms. Torrång, Ms. Tuvblad, and Drs. Cnattingius and Lichtenstein are with the Department of Medical Epidemiology and Biostatistics, Karolinska Institutet; and Dr. Larsson is with the Department of Child and Adolescent Psychiatry, Karolinska University Hospital, Stockholm, Sweden.
authors.forename: Catherine
authors.initials: C
authors.lastname: Tuvblad
authors.affiliation: Dr. Hultman is with the Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Stockholm, and the Department of Neuroscience Psychiatry, Ulleråker, University of Uppsala, Sweden; Ms. Torrång, Ms. Tuvblad, and Drs. Cnattingius and Lichtenstein are with the Department of Medical Epidemiology and Biostatistics, Karolinska Institutet; and Dr. Larsson is with the Department of Child and Adolescent Psychiatry, Karolinska University Hospital, Stockholm, Sweden.
authors.forename: Sven
authors.initials: S
authors.lastname: Cnattingius
authors.affiliation: Dr. Hultman is with the Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Stockholm, and the Department of Neuroscience Psychiatry, Ulleråker, University of Uppsala, Sweden; Ms. Torrång, Ms. Tuvblad, and Drs. Cnattingius and Lichtenstein are with the Department of Medical Epidemiology and Biostatistics, Karolinska Institutet; and Dr. Larsson is with the Department of Child and Adolescent Psychiatry, Karolinska University Hospital, Stockholm, Sweden.
authors.forename: Jan-Olov
authors.initials: JO
authors.lastname: Larsson
authors.affiliation: Dr. Hultman is with the Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Stockholm, and the Department of Neuroscience Psychiatry, Ulleråker, University of Uppsala, Sweden; Ms. Torrång, Ms. Tuvblad, and Drs. Cnattingius and Lichtenstein are with the Department of Medical Epidemiology and Biostatistics, Karolinska Institutet; and Dr. Larsson is with the Department of Child and Adolescent Psychiatry, Karolinska University Hospital, Stockholm, Sweden.
authors.forename: Paul
authors.initials: P
authors.lastname: Lichtenstein
mesh_terms: D000293:Adolescent; D001289:Attention Deficit Disorder with Hyperactivity / Q000453:epidemiology*; D001724:Birth Weight; D002648:Child; D002675:Child, Preschool; D039721:Diagnostic and Statistical Manual of Mental Disorders; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D008297:Male; D011446:Prospective Studies; D013548:Sweden / Q000453:epidemiology; D014427:Twins