A new study from Australia sheds more light on what environmental factors
might raise the risk for attention-deficit/hyperactivity disorder (ADHD).
"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 labour, and were more likely to have
given birth slightly earlier," said study co-author Dr Carol Bower, a
senior principal research fellow with the Centre 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 labour. Previous research
had suggested its use during childbirth might actually increase the risk of
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 Centre.
Numbers have levelled off
"Many prior studies have found an association between ADHD and [tobacco
and alcohol exposure in the womb], prematurity and complications of pregnancy
and delivery," she said.
One thing is certain: Diagnoses of ADHD have become common in the United
States. A survey released in November found that 10% of American children have
been diagnosed with the condition, although the rapid increase in numbers seems
to have levelled 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 paediatric 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 foetus, 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 in the print issue of the journal Paediatrics.
Find out more on ADHD here.
For more about ADHD, visit the US National Library of Medicine.