The present study investigated the relations among fetal testosterone, child socio-emotional engagement and language development in a sample of 467 children (235 boys) from the Western Australian Pregnancy Cohort (Raine) Study. Bioavailable testosterone concentration measured in umbilical cord blood taken at birth was found to be significantly negatively correlated with socio-emotional engagement and vocabulary development for boys but not for girls. Socio-emotional engagement mediated the effect of boys' fetal testosterone levels on their vocabulary development. However, the size of the effects was small, and fetal testosterone and socio-emotional engagement were no longer significant predictors of boys' vocabulary scores after accounting for the effects of other predictors including maternal age and education, parity, and parent-child book reading. It is concluded that further research into these associations is warranted in both typical and atypical development and that this research would profit from including a broader focus on the role that proximal processes such as socio-emotional engagement, joint attention and imitation have in mediating the developmental effects of prenatal factors such as fetal testosterone exposure.