Is ADHD genetic?

Studies have found that people with ADHD have smaller brains in certain areas, including the prefrontal cortex, the striatum, the dorsal anterior cingulate cortex, the corpus callosum, and the cerebellum, compared to those without ADHD.²

A different study looking into how people with ADHD develop found a delay in cortical maturation. Cortical maturation refers to a series of significant neurodevelopmental events that allow for improved cognitive and behavioral function. This suggests that differences in your early development could affect your risk of ADHD in the future.³

Risk factors that are associated with the diagnosis of ADHD are thought to start as early as pregnancy. Prenatal simply means before birth, and is another way to refer to pregnancy. These risk factors could include:

• maternal overweight or obesity
• smoking during pregnancy
• heavy alcohol consumption during pregnancy
• maternal diabetes
• high blood pressure caused by pregnancy (also known as hypertensive disorders of pregnancy, or HPD)

One study followed the development of 734 preterm infants, who were screened for ADHD at age 10 and assessed at age 15 to evaluate for diagnosis of ADHD. The study revealed that high maternal BMI and diabetes were associated with a 55-65% increase in ADHD risk.

Interestingly, the study also found no link between either maternal smoking or high blood pressure during pregnancy with an increased risk of developing ADHD, contrary to prior studies. This could suggest that the increased risk of ADHD due to maternal hypertension or smoking isn’t as significant as other factors in babies delivered preterm. It could, however, just be that premature birth simply limits the baby’s exposure to the high blood pressure environment or toxins from the cigarettes.⁴

There was also some evidence that high levels of inflammatory proteins during pregnancy were associated with increased risk of ADHD diagnosis.⁴ Levels of inflammatory proteins during pregnancy (also called perinatal inflammatory protein) can be measured with C-reactive protein (CRP), which is a sign of inflammation. High CRP levels during pregnancy have been associated with lower fetal weight in the third trimester and at birth⁵ which is a postnatal factor that can increase risk of ADHD diagnosis later in life.⁶

Postnatal refers to the period after giving birth, and there’s evidence that factors surrounding birth can affect the risk of ADHD outcomes, too. For example, research suggests that physical factors like low birth weight and premature birth can make ADHD more likely.

One study assessing 483 children found that those diagnosed with ADHD were about three times more likely to have been born with low birth weight compared with controls, even after accounting for factors like prenatal smoking and parental ADHD.[10] And a meta-analysis including a total of 1787 people showed that babies born very preterm or with very low birth weight were about three times more likely to be diagnosed with ADHD than babies born at full term with normal weight.¹¹

However, a study on the development of 119 preterm children found that, while the proportion of preterm children with clinically significant ADHD wasn’t different from what was expected, around 33% of them met criteria for subthreshold ADHD – a condition with some core symptoms of ADHD, but that doesn’t meet the full clinical diagnosis. This is higher than the typical 20%, highlighting the importance of monitoring and supporting the development of preterm children – especially as subthreshold ADHD can lead to a full ADHD diagnosis in the future if left unsupported.⁷

And lastly, mental health factors like postnatal depression can have an impact on ADHD too.⁶ A study that collected data from 5,635 mother-child pairs found that, after adjustments for outliers, children born to mothers with depression during the perinatal period were over three times more likely to be diagnosed with ADHD.⁸