April 18, 2011 -- Researchers studying more than a million children in Sweden have found that babies who are born prematurely have an increased risk of developing attention deficit hyperactivity disorder (ADHD) in their school years, even when they are compared with their full-term siblings.
The risk for hyperactivity and attention problems appears to increase for children who are born a month or more before their due dates and gradually rises with each additional week of prematurity.
Babies born very early, between 23 and 28 weeks of gestation, had more than double the risk of developing ADHD compared with those who were carried to term, from 39 to 41 weeks.
Those born between 35 and 36 weeks of gestation, a much more common circumstance, had about a 30% greater chance of having ADHD compared with babies that arrived on time.
While that finding is not new, experts note that the study makes the association stronger because it found a way to address a nagging question: Are environmental or genetic factors associated with premature birth responsible for the link, or is being born too early the real culprit?
In addition to studying the risk for ADHD in the general population, the Swedish researchers were able to look at what happens between brothers and sisters.
“You’re comparing two siblings, one who was born on time, and one who was born early,” says Mark A. Klebanoff, MD, MPH, a pediatrician and epidemiologist at Nationwide Children’s Hospital in Columbus, Ohio.
“People have used this design in a lot of other areas. I’m not aware that anyone has used that to study ADHD,” says Klebanoff, who was not involved in the study.
“What makes that very clever,” he says “is we’re controlling for all the things that are constant within a family but that we may not measure very well. It’s controlling for a mother’s genes, and assuming that it’s the same father, then we’re also controlling for the father’s genes, and anything that might be constant about the mother’s environment during pregnancy or when the children are young.”