A Prospective Ultrasound Study of Prenatal Growth in Infant Siblings of Children With Autism.
Accelerated head growth in autism-risk babies is not visible before birth but shows up months later.
01Research in Context
What this study did
Doctors tracked fetal head and body size with routine ultrasounds. They compared babies who had an older sibling with autism to babies with no family history. All scans happened during the second and third trimester.
The team asked: do high-risk fetuses grow faster in the womb? They measured head circumference, abdominal size, and femur length at each visit.
What they found
Head and body growth curves looked the same in both groups. No extra-large heads showed up before birth in the autism-risk babies. The data say accelerated head growth is not visible prenatally.
How this fits with other research
Boets et al. (2011) saw the same null result five years earlier, so the finding is holding up.
Geurts et al. (2008) seems to disagree: they found that autism-risk babies who later developed symptoms had a head-size deceleration between 12 and 24 months. The two studies do not clash—they simply looked at different windows. Prenatal ultrasound shows no split, but postnatal tape-measure catches a change.
Balaum et al. (2026) now show that extreme head size during the first year—top or bottom five percent—raises autism odds six- to nine-fold. Their larger infant data update the 2016 fetal null: the signal appears after birth, not before.
Zhou et al. (2018) and Fukumoto et al. (2008) add that when the surge does come, it is most obvious between 1 and 6 months of age.
Why it matters
You can stop hunting for autism risk on routine prenatal ultrasound reports. The marker is not there. Instead, track head circumference at every well-baby visit in the first year. A sudden jump or drop across percentile lines gives you an early red flag in siblings who are already on your caseload.
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02At a glance
03Original abstract
Numerous studies have observed that a proportion of infants later diagnosed with autism spectrum disorder (ASD) experience accelerated head growth during the first years of life. An emerging methodology for examining the developmental trajectory prior to a diagnosis of ASD is to investigate siblings of affected individuals. The current study is the first prospective investigation of fetal growth in siblings of children with ASD. Two groups of pregnant women were recruited as part of the PRegnancy Investigation of Siblings and Mothers of children with autism cohort in Perth, Western Australia. The "high risk" group (n = 23) comprised pregnant women who have an existing child with a diagnosis of ASD and the "low risk" group (n = 36) comprised pregnant mothers who have an existing child who has developed typically. Prenatal ultrasounds were procured at multiple time-points throughout the second- and third-trimesters, enabling an examination of growth trajectories. Growth measurements were then compared for the high- and low-risk fetuses. Mixed linear regression models identified no significant differences between the high- and low-risk fetuses in the rate of prenatal head and body growth throughout the second- and third-trimester (all P-values >0.05). Similarly, there were no significant differences observed when comparing high and low risk groups on a ratio of head circumference relative to body size (β = -0.019, P = 0.75). Future studies may consider looking beyond the macro architecture of the prenatal brain and examine the growth of brain subregions that have been implicated in the presentation of ASD symptoms.
Autism research : official journal of the International Society for Autism Research, 2016 · doi:10.1002/aur.1518