While gender differences in mathematical competencies favoring boys are well researched for school and late Kindergarten age, much less is known about their earlier development. Using data from the NEPS Newborn Cohort study, this paper focuses on 4 year-old children and the effects of the numerical and literacy stimulation during mother-child interactions they received at age 2. After controlling for different domain-specific stimulations, structural characteristics of the children's home learning environment, parental views, important child characteristics, and general educational processes between mother and child, empirical analyses showed small but statistically significant gender differences in mathematical competencies favoring girls. Early domain-specific stimulation in mother-child interactions at age 2 did not offer a good explanation for this math advantage of girls over boys at age 4. Nonetheless, even when 2 year-old girls received numerical stimulation quite similar to that of boys, there was some evidence that girls received a higher input in terms of literacy stimulation, and this is related to the small advantage of girls in mathematical competencies at age 4.