Women who smoke during pregnancy are at much higher risk of giving birth to a child who will grow up to have ADHD than those who are non-smokers, according to a new review of research studies from the years 1998-2017.

The study, Maternal Smoking and Attention-Deficit/Hyperactivity Disorder in Offspring: A Meta-analysis, confirmed that the risk of ADHD rose with the intensity of the smoking by the mothers.

For mothers who smoked fewer than 10 cigarettes per day, the risk of giving birth to a child who developed ADHD was 54 percent higher than for non-smokers. For mothers who were heavier smokers, the risk was 75 percent higher than those who were non-smokers. The statistics were consistent across time periods and continents.

The findings of the meta-analysis by Dr. Dezhi Mu and colleagues at West China Second University Hospital in Chengdu, Sichuan, China, were published December 29 online in the journal, Pediatrics.

The researchers reviewed 20 studies published between 1998 and 2017 that focused on the potential impact of smoking during pregnancy on the developing fetus and the subsequent risk of ADHD in the offspring. The studies involved a total of nearly three million people across five continents, and were focused in the United States, Brazil, Japan, Australia and Europe.

The U.S. Centers for Disease Control and Prevention reports that 11 percent of U.S. children ages 4 to 17 – 6.4 million children – have been diagnosed with ADHD based on parent-response reports.

Approximately 10 percent of American women reported smoking during the final trimester in their pregnancies, according to 2011 Pregnancy Risk Assessment and Monitoring System (PRAMS) data from 24 states.

Data from seven studies showed a 20 percent higher risk of ADHD in children born to fathers who smoked, even though mothers’ smoking had a greater effect.

“The limitations of our study included different assessment tools of ADHD and a lack of objective biological measures for maternal smoking,” the researchers added.

“With our meta-analysis, we provide evidence for an association between maternal smoking and offspring ADHD but do not solve the causality issues concerning potential confounding by other risk factors.

“More high-quality studies are needed to establish whether the association with smoking is causal.”