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.
Within the Same Family, Prematurity and ADHD Risk
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.”
ADHD and Preterm Birth
For the study, researchers in Sweden pulled government records for more than a million school children who were born between 1987 and 2000.
They compared data about the children’s births and family lives to prescription drug records.
For the purposes of the study, kids were determined to have ADHD if they had been prescribed at least one stimulant in 2006.
Stimulant medications for ADHD in Sweden may only be prescribed by specialists who are familiar with the disorder.
Overall, 7,605 children had a record of an ADHD medication in the Swedish Prescribed Drug Register, representing about 1% of the boys in the study and about 0.3% of girls.
The researchers, who were from the Karolinska Institute and Uppsala University, tried to tease out the influences of well-known risks factors by controlling for things like maternal age, education, smoking status, single parenthood, government assistance, and a history of psychiatric disorders in the parents.
They also adjusted for low Apgar score, which measures the health of newborns, and small size at birth.
Researchers found that even for moderate prematurity, or birth between 33 and 36 weeks' gestation, and even between siblings, being born early was associated with an increased risk of ADHD.
Absolute Risk Still Low
While that sounds concerning, other researchers who have looked at this question say it’s important to remind parents that most babies born early won’t have any ADHD problems.
“That was something big we wanted to drive home,” says Nicole M. Talge, PhD, an epidemiologist at Michigan State University in East Lansing. Talge published a study in December in the journal Pediatrics that also found a link between ADHD and premature birth, though she says most kids who were born early didn’t have attention problems. “At least 70 to 80 percent had scores that were considered to be in the normal range.”