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.
METHOD: 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 (beta = -.42; 95% CI: -.53, -.30),
inattentive (beta = -.26; 95% CI: -.33, -.19), and hyperactive-impulsive (beta =
-.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.The Swedish Council for Working Life and Social ResearchThe Swedish Research CouncilManuscrip
