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"Compared with mothers whose children did not have ADHD, mothers of children with ADHD were more likely to be younger, single, smoked in pregnancy, had some complications of pregnancy and labor, and were more likely to have given birth slightly earlier," said study co-author Dr. Carol Bower, a senior principal research fellow with the Center for Child Health Research at the University of Western Australia. "It did not make any difference if the child was a girl or a boy."
The researchers did find that girls were less likely to have ADHD if their mothers had received the hormone oxytocin to speed up labor. Previous research had suggested its use during childbirth might actually increase the risk of ADHD.
The causes of ADHD remain unclear, although evidence suggests that genes play a major role, said Dr. Tanya Froehlich, an associate professor at Cincinnati Children's Hospital Medical Center.
One thing is certain: Diagnoses of ADHD have become common in the United States. A survey released in November found that 10 percent of American children have been diagnosed with the condition, although the rapid increase in numbers seems to have leveled off.
ADHD is more prevalent in boys. Its symptoms include distractibility, inattention and a lack of focus.
In the new study, researchers examined the medical records of nearly 13,000 children and young adults who were born in Western Australia and took stimulant medications for ADHD between 2003 and 2007. Stimulant drugs such as Ritalin and Adderall are typically used to treat ADHD.
The researchers compared the subjects to more than 30,000 other children to see if there were any environmental differences.
Although factors such as a mother's younger age and smoking during pregnancy were linked to a higher risk of ADHD in children, "low birth weight, birth at greater than full term and breathing difficulties in the baby were not more common [in the ADHD group]," Bower said.
What's going on?
"Chronic exposure to smoking in pregnancy may create an imbalance in chemicals that result in ADHD," said study lead author Desiree Silva, a professor of pediatric medicine at the University of Western Australia.
But Froehlich said the picture may be even more complicated.
Some researchers have suggested that "people with ADHD are more likely to smoke, and then may pass on their ADHD-related genes to their children," Froehlich said.
Urinary tract infections also are thought to contribute to inflammation that affects the development of the brain in the fetus, she said. Stress during pregnancy -- perhaps from being single or a young mother -- could do the same thing.
"[However], since ADHD is associated with higher rates of teen pregnancy, it is also possible that the younger and single mothers themselves have higher rates of ADHD, and they are passing on their ADHD-related genes to their children," Froehlich said.
The Australian researchers called for more study on the subject.
The study appears online Dec. 2 and in the January print issue of the journal Pediatrics.
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