Little is known about differences in child developmental vulnerability before school entry according to maternal birthplace and sex. Official immigration records were linked with the Early Development Instrument assessments among children in kindergarten in the province of Manitoba, Canada (2005-2017). Logistic regression was used to estimate odds ratios of vulnerability in five developmental domains associated with maternal birthplace and child sex. Children of immigrant mothers from most birthplaces had higher adjusted odds of developmental vulnerability than non-immigrants in domains related to language and communication skills, except those of the rest of North America & Oceania. Children of Sub-Saharan African mothers were more vulnerable in four domains. Boys were consistently more vulnerable than girls across domains and maternal birthplaces. Children of immigrant mothers exhibited higher developmental vulnerability than non-immigrants in domains related to language and communication skills, potentially reflecting exposure to English and French as second languages.