Boys with autism spectrum disorder (ASD) are born smaller and lighter than average but outgrow peers by the time they are three. That's according to a new study published in the April edition of Autism Research, apparently the first to link ASD to rapid skeletal growth.
A team of researchers from the La Trobe University in Melbourne, Australia, compared the growth charts of 134 boys with ASD against those of 74 boys who were neurotypical. In particular, they examined height, weight, and head circumference, which is taken as standard practice in Australia every few weeks after birth and when the infant is 5, 8, 12, 18, and 42 months old. To avoid any bias, they excluded any who were born premature or who had been prescribed growth medications.
The researchers found that the average baby boy with ASD was born with a smaller head circumference than the average baby boy without ASD – typically, the study shows, 1.2 centimeters (0.5 inches) smaller than average. They were also born 4.8 centimeters (1.9 inches) shorter and 0.2 kilograms (0.4 pounds) lighter.
Interestingly, by the time they reached their third birthday, the infants with ASD frequently outgrew their peers. While their weight gain did not seem particularly affected by the disorder, they did appear to grow at a faster rate than neurotypical children.
The researchers are not really sure why this is. Cherie Green, a research fellow at La Trobe University and co-author on the study, suggests it could be a growth-hormone imbalance.
It's worth mentioning that it was a relatively small-scale study and that these results are not backed up by an older study, which in 2014 found no differences in the rate of growth between children with and without ASD. There is also the fact that no girls were involved in the study and, as other research has shown, autism manifests very differently between the two sexes. It could be that this is a condition that affects children with certain subsets of ASD, Green adds.
“Once you sort of start to understand a bit more about the biology, it might lead us to identifying particular subgroups,” Green told Spectrum, speculating that people with mutations in the gene CHD8 may have a larger head whereas those with a mutation in the gene DYRK1A may have a smaller head.
ASD is a disorder that affects more than 1 percent of the population and this type of research could have important consequences in determining how other mechanisms outside the brain may be involved, Green added. The next steps are to examine growth in adolescents to determine what – if anything – it says of the severity of various autistic traits.