October is ADHD Awareness Month, making it the perfect time to tackle a topic I’ve been really looking forward to writing about: the surprisingly high correlation between preterm birth and ADHD, or attention-deficit hyperactivitiy disorder (which used to be known as ADD or attention deficit disorder).
Numerous studies list prematurity and low birth weight as “risk factors” associated with developing a diagnosis of ADHD, or attention-deficit hyperactivity disorder, as it’s currently known in the DSM / Diagnostic and Statistical Manual. Some of Google’s top-level links include suggested questions like “Can prematurity cause ADHD?” A page on Hand to Hold, an organization supporting parents of preemies, about prematurity and ADHD offers a helpful overview of what ADHD looks like in children, and notes that recent studies show that being born premature, or of low birth weight, increases a child’s risk of developing ADHD.
The medical community currently recognizes three main types of ADHD:
1. ADHD, primarily inattentive (sometimes still referred to as ADD, or “attention deficit disorder”
2. ADHD, primarily hyperactive-impulsive type (this is the stereotype of the little boy who can’t sit still
3. ADHD, combined type (which includes both inattention and hyperactive/impulsive symptoms)
Currently, ADHD is thought to affect between 3-10% of children, and it’s generally diagnosed 3 times more frequently in boys than in girls, according to ADDitude Magazine. (This is only slightly less shocking than the 4-to-1 girls-to-boys ratio in autism diagnoses).
Statistics on ADHD and Children Born Preterm
Research on prematurity and ADHD returns even more startling statistics than the ADHD gender gap, however. One case in point, a 2011 Swedish study (lead author, Anders Hjern) drawing on official records of over 1.8 million individuals, notes that the risk of developing ADHD symptoms increases with each increasing week of prematurity (source: Journal of Pediatrics).
- The authors suggest that even babies born as late as 37 or 38 weeks gestation – who would technically be considered full-term – still have a 20% increased risk of developing ADHD symptoms compared to babies born at a full 40 weeks (source).
- Infants born at 35 or 36 weeks preterm — the gestational age of most preemie infants — are at a 30% increase in risk for ADHD compared to full-term babies.
- Every week matters: the earlier a baby is born, the more likely she or he is to develop ADHD-like symptoms.
Time Magazine’s writeup of the 2011 Swedish study makes the case explicit:
The risk increased with the degree of prematurity: babies born at 33 to 34 weeks were 40% more likely to have ADHD than full-term babies, and those born at 29 to 32 weeks were 60% more likely. Babies born even earlier than that — between 23 and 28 weeks — had twice the risk. (Source: Time. Emphasis added.).
ADHD Manifests as Inattention in Former Preemies
Other studies dig deeper, revealing that among former preterm babies who develop either ADHD symptoms or ADHD itself, preemies show far more inattentive-type behavoirs than impulsive or hyperactive behaviors. A 2011 study in the journal Pediatric Research (“Preterm Birth and Childhood Psychiatric Disorders,” by Samantha Johnson and Neil Marlow), for example, described the ways in which what they identified as a “preterm behavioral phenotype” includes a striking resemblance to certain aspects of both autism and ADHD. Their research focused on children born prior to 32 weeks gestational age, or weighing less than 3 lbs, 5 oz. (very low birthweight). Findings relevant to ADHD revolved around several key points:
- the male presentation of hyperactivity and impulsivity is largely lacking in preterm children who exhibit ADHD symptoms
- former preemies also lack the bad behavior sometimes associated with ADHD (referred to in the article as “comorbid conduct disorders”)
- there is a greater risk of symptoms of inattention in very preterm (before 32 weeks) or very low birth weight (smaller than 3 lbs 5 oz, or 1,500 grams) (source: see page 4 in the study referenced here).
Looked at from the other direction, the authors of the study noted that characteristics associate with ADHD-primarily inattentive resemble general traits found in former preemies: shyness, social withdrawal, lack of aggression, academic difficulties, and deficits in working memory or processing speed.
Finally, and this is key, the researchers note two very important points, which will lead us to Parts 2 and 3 of this series on ADHD and Prematurity:
1. Symptoms of ADHD or inattention in former preemies derive from a “different causative pathway” than for ADHD in the full-term population. In other words, ADHD in former preemies is thought to be caused by changes in brain development as a result of being born early (that’s Part 2 of this series! Stay tuned!), whereas the brain differences found in non-preemie ADHD are more likely caused by inherited traits.
2. Symptoms of inattentive ADHD may sometimes present as subclinical (so, subtle differences from the norm that may not be not outright impairing, but are still present), but which may still benefit from psychiatric or neuropsychological screening. (Ask me how I know this one, and stay tuned for Part 3 of this series, what does all this look like in the life of an actual former preemie.)
Like what you’re reading? I hope so, because there’s more to come…
This is Part 1 in a several-part series about prematurity and ADHD for October, ADHD Awareness Month.
Part 2 of the series takes a closer look at the brain, asking: “What regions of the brain are neurodivergent/neuroatypical in prematurity and in ADHD? Why is ADHD so much more common in former preterm children or adults than in the full-term population?”
Part 3 turns to the question of “what does all this look like in the lives of former preterm infants?” If it’s not quite ADHD, but still gets in the way of functioning, what do we call it? Following the authors of thes tudy mentioned above, who refer to a “preterm behavioral phenotype,” I call it the preemietype… Stay tuned!
The Tl;dr [Too long; didn’t read]:
1. Preemies have a greater likelihood of developing ADHD than full-term babies.
2. The earlier a baby is born, the greater the likelihood of having ADHD-like symptoms.
3. Preemies tend to develop symptoms closest to the “inattentive” subtype of ADHD, which is remarkably close to what researchers identify as a “preterm behavioral phenotype.”
4. I’ll be writing more about ADHD and prematurity in October for ADHD Awareness; subscribe to this blog, or follow on Facebook or Twitter, to keep updated!