Head circumference trends in autism between 0 and 100 months

Meta-analyses of head circumference in autistic individuals exist; however, simple meta-analytic approaches are limited. Consequently, we gathered head circumference raw data of autistic ( N = 2381) and typically developing participants ( N = 994) by re-analysing the data from previously published studies together. The present study found no mean difference between head circumference of autistic and typically developing individuals, although simple effect analyses revealed smaller mean head circumference in autistic than typically developing females aged 12–17 months. However, compared to controls, the frequency of extreme head circumference in autistic males was greater at birth and between 60 and 100 months. In addition, the frequency of extremely small head circumference between 6 and 11 months, and extremely large head circumference between 12 and 17 months, was greater in autistic than typically developing males. For autistic females, compared to controls, extreme head circumference was more frequent between 36 and 59 months and less frequent at birth. We conclude that it is imperative to consider the effects of age and sex when investigating the relationship between autism diagnosis and head circumference. This variance was more effectively described via the approach of the present study than previous meta-analytic approaches. Lay abstract Summaries of studies that have measured head size in those with autism, known as meta-analyses, currently exist. However, this approach does not adequately explain extreme cases (such as those with extremely small, or extremely large, head size). Because of this, we obtained all available published data measuring head size (12 studies). The data from each study were then combined to make a larger dataset. We found that females with autism aged 12–17 months had, on average, smaller head sizes. Otherwise, average head size was not atypical in autism. However, we found that males with autism were more likely to have extreme head sizes at birth and between 60 and 100 months, a small head between 6 and 11 months, and a large head between 12 and 17 months. Females with autism were more likely to have extreme head sizes between 36 and 59 months and were less likely at birth. Our approach was able to measure the influence of age and biological sex on head size in autism, as well as the frequency of extreme cases of head size in autism. These results add to what we already know about head size in autism.