The literature reports the existence of a series of events in language acquisition, corresponding to different stages, as children advance from basic to more complex grammar structures when learning their first language. With the greater complexity of the syntax context, children should produce phrases where prepositions appear. Thus, the syntax context should be one of the prerequisites for the appearance of these grammatical elements in their speech. To verify this hypothesis, our study collected recorded samples of children's language use in familiar contexts and with familiar interlocutors. The sample consisted in 16 children aged 18 to 36 months; and the sample was divided in four groups corresponding to the subject's age group (18, 24, 30 and 36 months). The results indicate that prepositions appear in more complex contexts when the child has learned the basic grammar of his/her first language, which occurs between 18 and 24 months of age.