03/23/2017 11:54 EDT | Updated 03/23/2017 11:54 EDT

What's Causing The Increased Rate Of ADHD?

Close Up Of Pregnant Woman Smoking Cigarette
Highwaystarz-Photography via Getty Images
Close Up Of Pregnant Woman Smoking Cigarette

The rate of ADHD diagnosis in children, teens and adults is increasing. In the United States, the Center for Disease Control documents that in 2003, 7.8% of children had ever been diagnosed with ADHD. In 2011, the rate had gone up to 11%. There are also recent increases in teens and adults diagnosed with ADHD. With such a significant increase in diagnosis, people wonder: Why is the rate of ADHD going up?

There are common theories about why ADHD is diagnosed more often. They include: More awareness of the condition, better recognition and diagnosis, pharmaceutical marketing, educational reforms and more. But recent research may hold a suprising answer. It relates to smoking during pregnancy.

Maternal smoking during pregnancy causes an increased rate of ADHD in their children. The research suggests that nicotine harms a baby's developing brain. The impact on the baby's brain makes it more likely to be diagnosed with ADHD later on. This is an environmental cause of ADHD, and suggests we could lower the risk of ADHD by helping pregnant mothers to quit smoking.

But that may not actually solve the problem...

New research is showing that smoking during pregnancy leads to a multi-generational change in risk for ADHD. This risk is transmitted through the mother's side of the family. In other words, if the maternal grandmother smoked during pregnancy, then her child and her grandchild is at higher risk of developing ADHD. Cigarette smoking's effect on brain development reaches down the generations, beyond just one.

Realizing that smoking was common in the twentieth century, many of our parents and grandparents smoked. Mothers didn't know they needed to stop smoking during pregnancy. Doctors didn't realize that smoking in pregnancy could increase the risk of ADHD in the child. They certainly didn't realize that smoking during pregnancy could lead to a multi-generational risk in the occurrence of ADHD. Smoking mothers 2 generations ago contributed to the current increased levels ADHD, without knowing it.

When we look at current smoking rates, they are lower than they were in the twentieth century, but a significant number of people still smoke. Research in ADHD shows that teens and adults with ADHD are two times more likely to smoke cigarettes than non-ADHD individuals. We also know that ADHD is highly heritable - i.e. it is passed genetically through families. When we combine the genetic risks for ADHD with the fact that adults with ADHD are more likely to smoke cigarettes, there are significant risks here.

Public health measures can support women to stop smoking before or during pregnancy. This may lower the rate of ADHD in their children. It will also help their grandchildren - in other words, the multi-generational impact of smoking can be lowered.

ADHD is a complex neurodevelopmental condition. Its causes are not fully known. Cigarette smoking in pregnancy gives us an environmental cause of ADHD which affects a child's brain development. It doesn't provide the whole answer to why the rate of ADHD is increasing, but it is still an important one.

Follow HuffPost Canada Blogs on Facebook