Preterm babies, babies born at least three weeks early, have long been known to be at higher risk of developing attention deficit hyperactivity disorder, and new research from Norway adds that infants born just one month premature are also at heightened risk.
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For the new study, recently published in the journal JAMA Pediatrics, researchers from Norway's Institute of Public Health analyzed data on 113,000 children, including 33,081 siblings.
“To our knowledge, this is the first study to investigate the association between preterm birth and symptoms of ADHD using a sibling-comparison design,” study authors wrote.
Researchers spoke to mothers about symptoms of ADHD when their children were at ages five and eight and compared the kids’ chances of developing the disorder with how premature they were.
They found that infants born at less than 34 weeks were at a greater risk of developing symptoms.
“Early premature birth was associated with inattentive but not hyperactive symptoms in 8-year-old children,” study authors wrote. “This study demonstrates the importance of differentiating between inattention and hyperactivity/impulsivity and stratifying on sex in the study of childhood ADHD.”
While ADHD is more prevalent in boys than in girls, a larger proportion of girls display inattentive symptoms, according to the study, which noted that being born preterm is associated with inattention more than with hyperactivity.
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Researchers suggest this link between preterm birth, ADHD and inattention is due to “the immaturity of the brain and its development.” At week 35 of gestation, the brain is in a critical period of development in utero.
For preterm babies born less than 34 weeks early, “this could lead to volumetric losses in specific brain regions and may partially explain the cognitive abnormalities in these children,” they wrote.
Limitations of the research include a 41 percent participation rate, suggesting bias with nonrandom participation; maternal reports that aren't equal to psychiatric evaluations; lack of representation of young women, smokers or women with low educational level and more.
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According to the Atlanta-based Centers for Disease Control and Prevention, 1 in 10 babies born in the United States is born too early, before 37 weeks of pregnancy have been completed.
The preterm birth rate rose for the second straight year in 2016 and the rate is about 50 percent higher among black women (14 percent) than among white women (9 percent).
ADHD, one of the most common neurodevelopmental disorders of childhood, affects about 6 million children in the U.S. And according to the CDC, about 2 million of those children were diagnosed at 2-5 years of age.
Symptoms of ADHD, which include forgetfulness, fidgeting, daydreaming or risk-taking, can cause difficulty through adulthood, with friends, at school and at home.
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