Brief report: a preliminary study of fetal head circumference growth in autism spectrum disorder.

Doctors measured fetal head size with routine ultrasound scans. They compared babies later diagnosed with autism to matched controls. The team looked at scans taken during pregnancy and at birth.

Head size did not differ between the two groups at any point. A few individual babies with autism had large heads, but the trend was not reliable. The authors conclude fetal head circumference alone cannot flag autism risk.

Stewart et al. (2018) ran a larger follow-up and also found no steady link between fetal head size and later traits. Eussen et al. (2016) repeated the design in autism-risk siblings and again saw no prenatal split, backing the null result.

Balaum et al. (2026) extends the story: head size becomes useful only after birth. They tracked infants every month for a year and found babies who stayed in the top or bottom 5 % carried much higher autism odds.

Cederlund et al. (2014) seems to clash, reporting only 3 % macrocephaly in preschoolers with autism—equal to the general rate. The gap disappears when you note the target looked at birth while Mats looked years later; large heads are not a constant feature across development.

Stop using a single ultrasound head measure as an early screener. The data say it adds no value prenatally. Instead, plot head growth across the first year if you need a physical marker, and pair it with social-communication checks. Share this with pediatricians so families get sound expectations and avoid needless worry.