"There has been quite a lot of research on very pre-term birth and the increased risk for ADHD but less evidence when it comes to late pre-term birth (weeks 34-36) and even less regarding babies born early term (weeks 37-38)," said lead author Dr. Minna Sucksdorff of the University of Turku in Finland.
In premature babies, the brain is still developing, and whatever caused the premature birth, like an infection in the mother, may have affected the brain, Sucksdorff told Reuters Health by email.
Researchers used three Finnish health registries to identify 10,321 children diagnosed with ADHD who had been born between 1991 and 2005. They compared each child with ADHD to four children without ADHD who had a similar birth date, gender and place of birth.
The registries included data on gestational age at birth, which was calculated using the mother's last menstrual period and first trimester ultrasound. Forty weeks is considered "full term."
Based on standards for each week of gestational age, the researchers noted which babies had been born of average weight, smaller than average or larger than average.
Mother's lifestyle plays an important role
Mother's age, substance abuse and smoking during pregnancy, number of previous births, marital status, father's age, and the urbanity of the child's birthplace were associated with gestational age, birth weight and ADHD, the authors reported in Pediatrics.
Accounting for other factors, premature birth was still associated with ADHD, with the risk increasing steadily as gestational age decreased.
Babies born at 25 weeks of gestational age were more than five times, or 500 percent, more likely to be diagnosed with ADHD as those born at 40 weeks.
At 38 weeks, babies were about 12 percent more likely to develop ADHD than those born full term.
Infants born significantly small or large for their gestational age also had an increased risk of ADHD.
This is not surprising, said Guilherme Polanczyk of the University of Sao Paulo Medical School in Brazil, as there is good evidence showing that prematurity and poor fetal growth are associated with a variety of chronic disorders such as high blood pressure, cardiovascular disease, diabetes, and mental disorders, including ADHD.
The fact that even some early term babies, who would not be considered premature, had an increased risk may indicate that the risk is continuous, with no specific cut-off point, he said by email.
"It is very interesting that those big for gestational age also have an increased risk," he said. "This is consistent with a large body of evidence showing that deviations of brain development may also occur in this population, probably because of different disease mechanisms."
Polanczyk was not part of the Finnish study.
"It is more and more established that prenatal health is very important also for mental, emotional, and cognitive development," Polanczyk added. "Preventing preterm birth is always an important goal, because it has a causal role to a variety of other negative outcomes."
Identifying children with increased ADHD risk may improve early detection and intervention, which can help reduce the adverse outcomes of ADHD, he said.
According to Sucksdorff, about five percent of children worldwide are diagnosed with ADHD.
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