Autism is a prenatal disorder: Evidence from late gestation brain overgrowth.
Autism-linked head overgrowth begins after 22 weeks in the womb, so late-pregnancy scans could give clinicians an early heads-up.
01Research in Context
What this study did
Doctors looked back at ultrasound scans taken during pregnancy. They compared head size in babies who later received an autism diagnosis with typical babies.
The team found that head growth sped up after 22 weeks gestation in the autism group. This points to a narrow prenatal window when brain development shifts off track.
What they found
Overgrowth starts in the womb, not after birth. The jump in head size is clear on routine third-trimester scans.
Because the change happens late in pregnancy, early delivery or imaging could flag risk months before behavioral signs appear.
How this fits with other research
Xiao et al. (2014) followed up with MRI scans at ages 2–3. They showed that the same children still had enlarged brains, proving the overgrowth persists into toddlerhood.
Talmi et al. (2020) linked lower birth weight and shorter gestation to higher autism odds. Their birth-record study widens the timeline, showing that even small deviations near the end of pregnancy matter.
Atladóttir et al. (2016) seemed to disagree at first glance. They reported that the extra autism risk from preterm birth has shrunk since the 1980s. The key difference is scope: Ó et al. tracked population trends, while Fréderique et al. zoomed in on individual head-growth curves. Both can be true—overall risk may drop, yet late-gestation overgrowth still signals autism in specific babies.
Why it matters
You can’t change prenatal growth, but you can use the timeline. When a toddler presents with early signs, ask parents about third-trimester ultrasound results. A sudden jump in head size after 22 weeks adds evidence that the child’s autism has neurobiological roots and may help families understand the diagnosis. Share the finding with pediatricians so they watch head-growth charts more closely, and keep an eye on any baby born after late-gestation overgrowth for early red flags.
Want CEUs on This Topic?
The ABA Clubhouse has 60+ free CEUs — live every Wednesday. Ethics, supervision & clinical topics.
Join Free →Pull the prenatal ultrasound report for any new toddler client and note head-size percentiles after 22 weeks gestation.
02At a glance
03Original abstract
This retrospective study aimed to specify the critical period for atypical brain development in individuals with autism spectrum disorder (ASD) using prenatal and postnatal head growth parameters. The sample consisted of 80 Caucasian, unrelated, idiopathic patients with ASD born after 1995. Fetal ultrasound parameters (head circumference [HC], abdominal circumference, and femur length) were obtained during the second and third trimesters of gestation. HC at birth and postnatal parameters at 12 and 24 months of age were also collected. Head overgrowth, assessed by HC, was highlighted during the second (20-26 weeks of amenorrhea) and third (28-36 weeks of amenorrhea) trimesters. Normal growth of body fetal parameters indicated that head overgrowth was not because of overall body overgrowth. Moreover, postnatal results replicated previously and reported head overgrowth. A critical time window for atypical brain development in autism is hypothesized to begin from the 22nd week of amenorrhea. This period is critical for cortical lamination and glial activation. A pathophysiological cascade is suggested with interactions between candidate genes and environmental factors. Autism Research 2018, 11: 1635-1642. © 2018 International Society for Autism Research, Wiley Periodicals, Inc. LAY SUMMARY: It is now widely acknowledged in the scientific community, that autism is a neurodevelopmental disorder. Recent evidence from animal and pathological studies has implicated the in utero period. However, the precise time of onset of abnormal brain development remains unknown. This retrospective study reports novel findings, identifying an atypical head growth trajectory in children with autism, during the in utero period (after the 22nd week of amenorrhea). In the same children, postnatal head overgrowth was also observed. Late gestation is identified as a critical period for atypical brain development underlying autism symptoms.
Autism research : official journal of the International Society for Autism Research, 2018 · doi:10.1002/aur.2036