The relation of low birth weight to the risk of attention deficit hyperactivity disorder was determined in a population-based sample of 1,480 twin pairs ascertained from the Swedish twin registry in the period 1985-1986. Eighteen twin pairs at 8-9 years of age and 10 twin pairs at 13-14 years were discordant for both birth weight and ADHD criteria. The child with ADHD was smaller in 11 of 18 pairs at 8-9 years and in 9 of 10 pairs in adolescence (P=0.011). The lighter twin had on average 13% higher ADHD symptom score at age 8-9 years (P=0.006) and 12% higher ADHD score at age 13-14 years (P=0.018) compared with the heavier twin. Similar effect sizes were found in MZ and DZ twins, and the associations did not diminish when genetic influences were controlled. Fetal growth restriction has a modest but significant environmental influence on the development of ADHD. 
COMMENT. Low birth weight and fetal growth restriction are risk factors for the development of ADHD in childhood, and the association is independent of genetic and other environmental influences such as maternal smoking and exposure to drugs during pregnancy.
Effect of long-term treatment with stimulant medication on growth of ADHD children. The reports of retarded growth of ADHD children treated with stimulants have been questioned in two recent studies (Pliszka SR et al, 2006; Spencer TJ et al, 2006) showing minimal or no significant effects on growth. A letter to the editor  points out that studies on growth should include treatment naive patients, because growth velocities progressively normalize with prolonged treatment. This criticism could be applied to the Pliszka study in which the majority of the cohort were previously treated, but not to that of Spencer in which absence of growth effects was not related to previous stimulant exposure. In an earlier uncontrolled study, an analysis of heights of 50 children failed to confirm a growth suppressant effect of methylphenidate, when conservative doses were employed and treatment was interrupted at weekends and on vacations (Millichap JG. Attention Deficit Hyperactivity and Learning Disorders. Chicago, PNB Publishers, 1998; 175-6).