Maternal smoking and the risk of ADHD

Alan S. Brown, MD, MPH

A recent study has strengthened the link between maternal smoking and later development of attention-deficit/hyperactivity disorder (ADHD) in children, but it is also the first to show that heavier smoking may increase these odds as well.

Previous studies have linked maternal smoking while pregnant to development of ADHD, but those reports relied on maternal self-reporting. The recent study, published in Pediatrics, measured actual levels of cotinine in the mother’s blood while she was pregnant.¹

Roughly 7.2% of women who gave birth in the United States in 2016 self-reported smoking cigarettes during their pregnancy, but studies have shown that self-reporting underestimates true smoking prevalence by 8% to 28%. Maternal smoking during pregnancy has been associated with a number of risks to infants, including low birth weight, sudden infant death syndrome, and childhood infections. Numerous studies also have linked maternal smoking in pregnancy to ADHD, but those studies relied on self-reporting, which may have underestimated the relationship between smoking during pregnancy and a later ADHD diagnosis.

This study used cotinine levels rather than self-reporting to determine maternal smoking habits. Cotinine is the byproduct of nicotine after it is metabolized in the liver. It stays in the body longer and can also enable researchers to quantify nicotine exposure by the level-even detecting passive smoking or the use of nicotine replacement therapy.

Alan S. Brown, MD, MPH, professor of Psychiatry and Epidemiology at Columbia University Medical Center (CUMC), the New York State Psychiatric Institute, and the Joseph L. Mailman School of Public Health, and director of the Program in Birth Cohort Studies at CUMC, New York City, co-authored the report and says the study provides evidence that a mother’s smoking during pregnancy could be related to the later development of ADHD in their infant.

The study used a database in Finland of blood samples stored from pregnant mothers, then investigated which children went on to developed ADHD. Results from more than 1000 infants later diagnosed with ADHD were analyzed against their mother’s blood samples from pregnancy, comparing them against an equal number of controls. Researchers found that the mothers whose children were later diagnosed with ADHD had a mean cotinine level that was more than double the level in mothers whose children were not diagnosed with ADHD.

Other factors also were analyzed in the study, including socioeconomic status, drug use, and mental health history of the mothers. The study notes, however, that even after adjusting for these factors, increasing cotinine levels in the mothers’ blood was associated with an increased risk for ADHD development in their children.

Cotinine dose-response is significant

The study is also significant in that it shows a dose-response to cotinine levels, with the likelihood of ADHD development increasing with higher maternal cotinine levels. According to the report, when the cohort was split into 3 groups based on cotinine levels, the mothers in the highest cotinine level group were 2.21 times more likely to have a child diagnosed with ADHD. When the cohort was split into 10 groups, mothers in the highest cotinine group became 3.34 times more likely to have a child diagnosed with ADHD.

Previous studies in animal models have shown that nicotine affects developing brain circuits, and is a chemical that can cross the placenta, according to the report. The result in animals of nicotine exposure is increased motor activity. Brown says more work is needed to understand the exact mechanism of how nicotine exposure relates to ADHD development.

“It would be important to understand factors that interact with nicotine and other chemicals in cigarette smoke, such as genes that increase the risk of ADHD,” Brown says. “Also, preclinical studies can tell us more about the brain mechanisms affected by smoking, which might provide insights into the pathways that lead to ADHD.”

Brown says he hopes the results of the study will give providers another tool when counseling mothers against smoking during pregnancy.