Women with low levels of serotonin a brain chemical associated with positive feelings are at an increased risk of having a child with attention deficit hyperactivity disorder, a new study suggests.

Children whose mothers had lower-than-normal serotonin levels were 1.5 to 2.5 times more likely to have ADHD than children whose mothers had normal serotonin levels, the study said.

"We have shown a new possible mechanism in the development of ADHD and a new kind of genetic pathway," said study researcher Dr. Anne Halmoy of the University of Bergen in Norway.

Past studies have shown a link between ADHD and decreased levels of serotonin in the person who has it, Halmoy told MyHealthNewsDaily. The chemicalis known to play a role in neurological development. The new study shows that ADHD is also associated with the mother's serotonin levels, she said.

Halmoy and her colleagues examined 459 adults with ADHD, 97 of their family members and 187 people without ADHD. They found 38 adults who had the changes in the enzymes needed to make serotonin which means they had lower-than-normal serotonin levels and looked to see whether their 41 children had ADHD.

Kids whose fathers had low serotonin levels weren't at a higher risk of developing ADHD than kids whose fathers had normal serotonin levels, the study said.

But the findings don't mean mothers-to-be should be overly concerned with their serotonin levels, Halmoysaid, because the amount of serotonin in the body is dependent on a number of complex and uncontrollable factors, including the environment, genetics and disease.

It's also unrealistic to get testing done to make sure you have "proper" serotonin levels, because simple blood tests don't accurately reflect how much serotonin is in the body, and because "today's knowledge is still too limited to allow conclusions about what an 'optimal' serotonin level is," she said.

The results are important but not unexpected, given what is already known about the relationship between a mother and child's serotonin levels, said Marc Caron, a biology professor at Duke University,who was not part of the study. He pointed to a 2006 animal study in the journal Proceedings of the National Academy of Sciences,whichshowed a mother passes on serotonin to her developing embryo.

Caron's own research in 1999 in the journal Science showed serotonin-regulating medications can have a calming effect on children with ADHD by normalizing chemical levels in their brains.

The study was published in the October issue of the journal Archives of General Psychiatry.