Women of Turkish and Moroccan origin in the Netherlands are relatively likely to have an unhealthy bodyweight. This study sheds light on how ethnic differences in parity, i.e., the number of times a female carried pregnancies to a viable gestational age, contribute to body mass index (BMI) differences between Turkish-born and Moroccan-born mothers aged 35 + and their native Dutch counterparts. We applied a Blinder-Oaxaca decomposition approach to pooled data from four migrant surveys based on national probability samples (n = 2,532). Unlike conventional mediation analyses, the Blinder-Oaxaca approach recognizes that the association between parity and bodyweight may vary across different groups. Our results indicated that Turkish-born and Moroccan-born mothers in the Netherlands had more children and a higher BMI than native Dutch mothers. Regression analyses moreover showed that the parity-BMI gradient was steeper for Turkish-born mothers than for native Dutch mothers. Decomposition using the Blinder-Oaxaca approach indicated that the higher number of children of Turkish-born and Moroccan-born mothers compared to native Dutch mothers contributed substantially to the higher mean BMI in the former groups. The steeper parity-BMI gradient in Turkish-born mothers further amplified the contribution of parity to the higher mean BMI of Turkish-born mothers as compared to native Dutch mothers. Future research is needed to assess to which extent the steep parity-BMI gradient in Turkish-born mothers can be explained by relatively strong barriers to a healthy lifestyle that Turkish-born mothers of a larger number of children may face due to a relatively strongly gendered division of household and childrearing tasks.