In the absence of intellectual impairment, girls are diagnosed with autism spectrum disorder (ASD) significantly less and later than boys. This study explored potential reasons for why ASD may be more difficult to early identify in girls, based on carer concerns during the pre-diagnosis period. Carers of 92 boys and 60 girls, diagnosed with ASD 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 preschool social situations, and the types of restricted interests. Findings suggest, from carer perspective, girls who went on to be diagnosed with ASD presented differently when compared to boys, providing insight in to why the diagnosis of ASD may be more difficult to make with cognitively-able girls.