Preterm Births Increase Risk of ADHD and Autism

Understanding the factors contributing to the prevalence of ADHD and autism requires examining the type and magnitude of risk factors for the disorders.  One risk factors common to both disorders is early or pre-term birth.  Advances in neonatal care dramatically increase survival in infants born as early as the fifth or sixth month of pregnancy.

A recent study published in the journal Pediatrics provides some important new data about the risk of ADHD in pre-term birth infants.  Scientists in Sweden examined the prevalence rates for ADHD in a group of children born preterm and compared it to the rates in infants born at term or slightly later.  Using several statistical models, they estimated the magnitude of effect of preterm birth on ADHD.  They found a significant statistical effect--the earlier the birth in gestation the greater the likelihood of childhood ADHD.

The magnitude of this effect is an approximate doubling of the risk for the earliest births.  The data from the study (model 3) estimated the risk (odds ratio) of childhood ADHD at several stages of development at birth:

• 23-28 weeks  Odds ratio=2.1
• 29-32 weeks  Odds ratio=1.6
• 33-34 weeks  Odds ratio=1.4
• 35-36 weeks  Odds ratio=1.3
• 37-38 weeks  Odds ratio=1.1
• > 38 weeks    Odds ratio=1.0

Similar studies in autism and autism spectrum disorder estimated a similar magnitude of risk for autism and autism spectrum disorder in preterm birth.  Infants born before 30 weeks appear to have approximately a doubled risk for autism and ASD.

From a prevention viewpoint, these studies support efforts to reduce the risk of preterm birth.  Although it is impossible to eliminate preterm births, the following measures can significantly reduce the risk:

• Smoking cessation during pregnancy
• Early and regular prenatal care
• Reducing the number of multiple birth pregnancies in infertility couples
• Progesterone use in women with a previous history of preterm birth
• Reducing the number of pregnancy inductions and C-sections in preterm pregnancies

Increased number of viable preterm births may be contributing to a portion of the number of children with ADHD and autism/autism spectrum disorders.  Improved neonatal care of preterm infants carries the potential for more children with special needs.  Improving neonatal care for infants in the neonatal intensive care as well as public health efforts to reduce the number of preterm births are critical initiatives for prevention of ADHD and autism.

The March of Dimes has an online resource site that provides professional education of the issue of reducing preterm births.

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Lindstrom, K., Lindblad, F., & Hjern, A. (2011). Preterm Birth and Attention-Deficit/Hyperactivity Disorder in Schoolchildren PEDIATRICS DOI: 10.1542/peds.2010-1279

Schendel D, & Bhasin TK (2008). Birth weight and gestational age characteristics of children with autism, including a comparison with other developmental disabilities. Pediatrics, 121 (6), 1155-64 PMID: 18519485

Autism Twin Studies Point to Key Next Steps

Angelica Ronald and Rosa Hoekstra have written a nice review of recent twin studies in autism spectrum and autism traits.  The review is online prior to print in the American Journal of Medical Genetics Neuropsychiatric Genetics.

They note in the review that twin studies have evolved from a perspective of looking at narrowly defined autism, to the broader category of autism spectrum to an even broader phenotype, the dimensional character of autistic traits in the general population.

I don't know much about autism traits in the general population but this twin studies review highlighted some current understanding.

• Relatives of individuals with autism spectrum disorder show elevated autistic traits
• Autistic traits in the general population show a smooth distribution through the normal range to the clinical extreme--this suggests a biological continuum rather than a normal or disease clinical status
• Understanding the cause of variation of autistic traits in the general population is likely to aid in the understanding of autism
• The heritability of autistic traits in the population is very heritable (genetic) ranging from 40 to 60% in young twins but increasing to 60 to 90% in adolescence.

Another insightful section of the manuscript dealt with the degree of genetic and environmental overlap between different autistic symptoms.  Autism spectrum is made up of a triad of symptoms: social impairment, communication impairment and restrictive behaviors and interests.  Twin studies suggest these different symptom clusters may be "fractionable", meaning they may have some independence from each of the other clusters.  The authors note more genetic and twin studies should focus on one symptom cluster of the triad, rather than only studying those who have all three components.

The authors note that twin studies have helped explain the relationship between autism and intellectual disability and low IQ.  There appears to be some common genetic contribution to autism and intellectual disability.  However, since some individuals with autism and ASD have relatively normal intelligence, the picture is complex.  Studying ASD in those with normal intelligence may provide further insight into specific autism genetic influences. 

What needs to be done in the future?

Need for twin studies to tease out genetic and environmental factors that explain the common co-occurence of autism, ASD and autism traits with conduct disorder, sleep disorders, antisocial behavior and depression.

More attention needs to focus on genetic and environmental effects on developmental change in ASD and autistic traits in children (only two current studies published) and in adults (zero studies published).

More research on age-appropriate measures of autism traits--measures appropriate for one developmental age may not be appropriate for other ages.

More studies are necessary to look at cognitive phenotypes in autism and autism traits .

Photo of St. Louis Cardinal Daniel Descalso Courtesy of Yates Photography

Ronald, A., & Hoekstra, R. (2011). Autism spectrum disorders and autistic traits: A decade of new twin studies American Journal of Medical Genetics Part B: Neuropsychiatric Genetics, 156 (3), 255-274 DOI: 10.1002/ajmg.b.31159