In the last 50 years, researchers have debated over the lexical or grammatical nature of children's early multiword utterances. Due to methodological limitations, the issue remains controversial. This corpus study explores the effect of grammatical, lexical, and pragmatic categories on mean length of utterances (MLU). A total of 312 speech samples from high-low socioeconomic status (SES) French-speaking children aged 2-4 years were annotated with a part-of-speech-tagger. Multiple regression analyses show that grammatical categories, particularly the most frequent subcategories, were the best predictors of MLU both across age and SES groups. These findings support the view that early language learning is guided by grammatical rather than by lexical words. This corpus research design can be used for future cross-linguistic and cross-pathology studies.