In the absence of intellectual impairment, girls are diagnosed with autism spectrum disorder significantly less and later than boys. This study explored potential reasons for why autism spectrum disorder may be more difficult to identify in girls, based on carer concerns during the pre-diagnosis period. Carers of 92 boys and 60 girls diagnosed with autism spectrum disorder from school age completed an online survey addressing concerns regarding the child's development during the pre-school years (pre-diagnosis). Significant sex differences were evident in key early concerns, as well as the strategies used to navigate pre-school social situations, and the types of restricted interests. Findings suggest, from carer perspective, that girls who went on to be diagnosed with autism spectrum disorder presented differently when compared to boys, providing insight into why the diagnosis of autism spectrum disorder may be more difficult to make with cognitively able girls.