Of the children with confirmed hyperkinetic disorder, nine out of 10 were boys, although the increased risk was basically the same for both genders, the researchers wrote. However, the small number of girls in the study may have been a factor, they said. It is possible that the central dopamine(Drug information on dopamine) system matures more lowly in males than in females thereby increasing the period of vulnerability, they speculated.
A strength of the study was its large population, which provided prospective information about each individual, eliminating the risk of parental recall. Adjustment for genetic factors was also an important advantage compared with previous studies, Dr. Linnet said.
The researchers noted that they used birth weight as a proxy measure of intrauterine growth retardation. However, they said, this measure is difficult and is not ideal and that therefore, the results on intrauterine growth retardation should "be taken with caution."
Until now, Dr. Linnet said, most studies of clinically verified ADHD were performed with small samples and children with extremely low gestational ages at birth (under 28 weeks). However, she said, the majority of preterm babies are born at higher gestational ages, and "our findings may therefore have more important public health perspectives."