Low birth weight or premature birth increases the risk of clinically verifiable hyperkinetic disorder (HKD), according to a new study.

The study found that children born at less than 34 weeks gestational age were three times more likely to develop HKD than children born at term, said Karen Linnet, PhD, of the Perinatal Epidemiology Research Unit in the Department of the Obstetrics and Pediatrics at Aarhus University Hospital in Denmark, and colleagues. Full-term children born with birth weights below 3,000 grams also had an increased risk of HKD.

Hyperkinetic disorder is the ICD-10 clinical correlate of DSM-IV’s ADHD, combined type. Study cases were acquired from the Danish Psychiatric Central Register, which includes records, dates, and diagnoses of all inpatient admissions and outpatient contacts at psychiatric departments in Denmark.

The researchers noted that precisely how pre-term birth and poor intrauterine growth affect the fetal brain to cause HKD is not clear. They said one possibility is that fetal hypoxia and hypotension injure the striatal complex of the basal ganglia and increase the number of dopamine receptors. Under-nutrition during fetal brain development may also have long-term effects on attention, learning, and memory, as animal studies have shown, according to Linnet et al.

Much prior research has focused on very premature (prior to 28 weeks) or very low birth weight babies – those born weighing less than 1,500 grams. Linnet’s work indicates that risk remains with children born at higher gestational age or slightly underweight.

Martin Stein, MD, a professor of pediatrics at the University of California, San Diego School of Medicine, said the study “shows that the relative risk persists even with mild prematurity, babies born at between 34 and 36 weeks.”

The researchers matched each of the 834 children with 25 randomly selected children of the same gender and age. Children with autism or other pervasive developmental disorders were excluded in the case study. Gestational age was recorded by midwives at the time of birth and recorded in the Danish Medical Birth Registry. Gestational age of 40 to 42 weeks and birth weight of 3,000 to 3.999 grams were used as references.

Stein said the study benefited from the available information from the national medical system with comprehensive records and cases based on true diagnoses.

The researchers said that “compared with children born at term, children with gestational ages between 34 and 36 completed weeks had an 80% increased risk of HKD, and children with gestational ages below 34 had a three-fold increased risk. Children born at term with birth weights between 1,500 and 2,499 grams had more than a twofold increased risk of HKD compared with children born at term with birth weights above 2,999 grams, whereas children with birth weights between 2,500 and 2,999 grams had a 70% increased risk.”

Single-parent families, poor social factors, and younger parental age were HKD risk factors as well as previous psychiatric admissions for the child and parental psychopathology, but adjustment for these factors did not change the outcomes.

Stein said most premature infants won’t develop ADHD, but also noted that “this study is a reminder of the importance of including a detailed prenatal and perinatal history when evaluating a child for possible ADHD.”

Prematurity takes on added significance with the release in July of a report for the Institute of Medicine noting that 12.5% of births in the United States were pre-term in 2005, a 30% increase over 1981 rates. The report cited socioeconomic, genetic, biological, and environmental factors as possible causes. For example, girls under age 16 and women older than 35 have a greater chance of pre-term delivery.

The report called for more research into the causes of pre-term births. The results of the study were published in the June issue of Archives of Disease in Childhood.

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