Maternal body weight, measured as Body Mass Index (BMI) and infection during pregnancy are established risk factors of multiple adverse birth outcomes that are also associated with intellectual disability (ID) in children, but little is known of the interaction of these two factors. This retrospective cohort study sought to explore whether BMI is an effect modifier in the association between maternal infection during pregnancy and ID of the child, and to make comparisons between categories of gestational infection. The study sample (n= 124,047 after exclusion) was derived from Medicaid administrative data. After preliminary analysis of interaction, stratified logistic regression analysis was performed to assess magnitude of the effect modification by BMI. After adjusting for confounders, there was evidence that BMI modified the relationship between infection and ID. Underweight and obese mothers who experienced both a general infection (GI) and a sexually transmitted infection (STI) during pregnancy had the highest odds of having a child with ID, when compared to mothers of the same weight category with no infection (ORs 2.76; 95% CI 1.68 -- 4.53 & 1.47; 95% CI 1.13 -- 1.91). Underweight mothers who experienced both GI & STI during pregnancy also had higher odds of having a child with ID compared to underweight mothers who had either STI or GI only (ORs 2.41; 95% CI 1.44 - 4.03 & 3.76; 95% CI 1.54 - 9.17). There were no differences between STI and GI categories across the BMI strata. These findings aligned with existing related literature, and contribute to understanding the complexities of the relationships between infection, maternal BMI, and ID.