Background Research indicates that structural differences exist in the brains of individuals who later display developmental conditions (e.g., autism). To date only a handful of studies have explored the relationship between fetal brain growth and later infant outcomes, with a particular focus on fetal head circumference (HC) as a proxy for brain development. These findings have been inconsistent. We investigate whether fetal brain measurements correlate with the emergence of autistic traits in toddlers. Method 219 singleton pregnancies (104 males and 115 females) were recruited at the Rosie Hospital, Cambridge, UK. A 2D ultrasound was performed at 12-, 20- and between 26-30-weeks of pregnancy, measuring head circumference (HC), ventricular atrium (VA) and transcerebellar diameter (TCD). 178 infants were subsequently followed up at 18-20 months of age and completed the Quantitative Checklist for Autism in Toddlers (Q-CHAT) to observe early autistic traits. Results HC was larger in males than in females in both the second and third trimester. There was a significant positive association between TCD size at 20 weeks and Q-CHAT scores at 18-20 months of age, found in both univariate and multivariate analyses, and this remained significant after controlling for sex. Conclusion There is a positive relationship between cerebellar (TCD) development at 20 weeks’ gestation and the later emergence of autistic traits (at 18-20 months of age). Atypical neurodevelopment may start prenatally. If replicated these findings could facilities early diagnosis and improved outcomes.