The findings, reported in the Journal of Pediatrics, suggest that the so-called Apgar score assigned to all newborns in the first five minutes of life may give some hint of a child's future risk of ADHD, a condition that involves attention problems and impulsive behavior.
A newborn's Apgar score is based on several physical signs, including breathing, heart rate and muscle tone. A score of 7 or higher is considered normal, with a 9 or 10 indicating that the baby is in the "best possible condition."
In the new study, researchers found that among more than 980,000 Danish children, the risk of developing ADHD climbed as Apgar scores dropped.
Compared with children whose scores had been a 9 or 10, those with a 5 or 6 had a 63 percent higher risk of ADHD. And those with an Apgar of 1 to 4 had a 75 percent greater risk.
In the U.S., surveys have found that nearly one in 10 school-aged kids have been diagnosed with the condition, according to the Centers for Disease Control and Prevention.
Exactly what the new findings mean is unclear. An abnormal Apgar score could reflect some sort of stress during pregnancy or birth -- like decreased oxygen supply -- that might contribute to ADHD development down the road, the researchers speculate.
"There may be a sharing of causes of low Apgar score and ADHD, but our analyses do not point (to) any specific factor here," Dr. Carsten Obel, one of the researchers on the work, told Reuters Health in an email.
A number of studies have linked preterm birth to an increased risk of ADHD, although the reasons aren't clear, noted Obel, of Aarhus University in Denmark.
Both preterm birth and a low Apgar score may be markers of less-than-optimal fetal development, he explained.
The findings are based on Danish national registry data for 980,902 children born between 1988 and 2001. Of those children, 8,234 were diagnosed with ADHD -- most of them boys.
Even after the researchers accounted for factors like preterm birth, family income and mothers' smoking and education levels, the risk of ADHD was higher among kids with Apgar scores below 7.
Still, the vast majority of children in the study were not diagnosed with ADHD, regardless of Apgar score. And it's not clear whether the Apgar-ADHD link might eventually have any practical implications.
In this study group, Obel said, the ability of a child's score to predict ADHD was not that strong.
"We do not find that the predictive value is so good that we routinely should inform parents who (have) a child with a low Apgar score about the higher risk of ADHD," Obel said.
Future studies, he said, could try to undercover the reasons for the link between Apgar score and ADHD. Large population studies might also be able to look at how combinations of factors -- like low Apgar score plus a family history of ADHD -- come into play.