Premature birth or low birth weight increase the risk of clinically
verifiable hyperkinetic disorder (HKD), according to a study based on Danish
medical and population registers.
Children born at fewer than 34 weeks gestational age were three times more
likely to develop HKD than children born at term, said Karen Linnet, Ph.D., of
the Perinatal Epidemiology Research Unit in the Department of the Obstetrics
and Pediatrics at Aarhus University Hospital, Aarhus, Denmark, and colleagues.
Full-term children born with birth weights below 3,000 grams also had an
increased HKD risk.
Hyperkinetic disorder is the ICD-10 clinical correlate of
DSM-IV's attention-deficit/hyperactivity disorder, combined type. The
study was published in the June Archives of Disease in Childhood.
Cases in the study were drawn from the Danish Psychiatric Central Register,
which includes records, dates, and diagnoses of all inpatient admissions and
outpatient contacts at psychiatric departments in Denmark.
Exactly how preterm birth and poor intrauterine growth affect the fetal
brain to cause HKD is unclear, said the researchers.
One possibility is that fetal hypoxia and hypotension injure the striatal
complex of the basal ganglia and increase the number of dopamine receptors.
Undernutrition during fetal brain development may also have long-term effects
on attention, learning, and memory, as animal studies have shown, according to
Linnet and colleagues.
Much previous research has focused on extremely premature (birth 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.
"This study shows that the relative risk persists even with mild
prematurity, babies born at between 34 and 36 weeks," said Martin Stein,
M.D., a professor of pediatrics at the University of California, San Diego
School of Medicine, in an interview with Psychiatric News.
Linnet and colleagues matched each of the 834 case children with 25
randomly selected children of the same gender and age. Children with autism or
other pervasive developmental disorders were excluded. 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.
The study benefited from the availability of a national medical system with
comprehensive records and cases based on true diagnoses, not just parental
reports, said Stein.
"Compared with children born at term, children with gestational ages
between 34 and 36 completed weeks had an 80 percent increased risk of HKD, and
children with gestational ages below 34 had a three-fold increased
risk," said the researchers. "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 percent increased risk."
Single-parent families, poor social factors, and younger parental age were
risk factors for HKD, as were previous psychiatric admissions for the child
and parental psychopathology, but adjustment for these factors did not change
Too Young or Too Small
Premature birth and low birth weight are risk factors for later development
of hyperkinetic activity disorder, say Danish researchers.
Maternal smoking is a known risk factor for HKD in offspring, but limiting
analysis to children of nonsmoking mothers still found that preterm delivery
"Most premature infants won't develop ADHD," said Stein."
But this study is a reminder of the importance of including a detailed
prenatal and perinatal history when evaluating a child for possible
Prematurity takes on added significance with the release in July of a
report for the Institute of Medicine noting that 12.5 percent of births in the
United States were preterm in 2005, a 30 percent increase over 1981 rates. The
report cited socioeconomic, genetic, biological, and environmental factors as
possible causes. Girls under age 16 and women older than 35 have a greater
chance of preterm delivery, for instance. Increased use of assisted
reproductive technologies may play a role, because multiple births are common
following infertility treatments. Three out of five sets of twins and nearly
all triplets born following such procedures arrive prematurely. The IOM report
called for more research into the causes of preterm births.
An abstract of "Gestational Age, Birthweight, and the Risk of
Hyperkinetic Disorder" is posted at<http://adc.bmjjournals.com/cgi/content/abstract/adc.2005.088872v1>.▪