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Birth weight and attention-deficit/hyperactivity symptoms in childhood and early adolescence: a prospective Swedish twin study.

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1
Dr. Hultman is with the Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Box 281, SE-171 77, Stockholm, Sweden. Christina.Hultman@ki.se

Abstract

OBJECTIVE:

To determine whether low birth weight increases the risk of attention-deficit/hyperactivity disorder (ADHD) in childhood and early adolescence.

METHOD:

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.

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