Gender dysphoria (GD) and autism spectrum disorder (ASD) are associated. In 49 GD children (40 natal males), we examined ASD risk factors (i.e., birth weight, parental age, sibling sex ratio) in relation to autistic traits. Data were gathered on autistic traits, birth weight, parents’ ages at birth, sibling sex ratio, gender nonconformity, age, maternal depression, general behavioral and emotional problems, and IQ. High birth weight was associated with both high gender nonconformity and autistic traits among GD children. Developmental processes associated with high birth weight are, therefore, likely to underlie the GD–ASD link either directly or indirectly. The present study is the first to provide quantitative data bearing on possible mechanisms that lead GD and ASD to co-occur.