In this work, the data on the features of speech development in children with autism spectrum disorders (ASD) and with Down syndrome (DS) are presented. Methods of perceptual experiment and instrumental analysis were used. It is shown that in typically developing (TD) children the values of vowel articulation index of stressed vowels in words are maximal in comparison with the corresponding characteristic in children with ASD and children with DS. The words of children with ASD are characterized by high pitch values of words and stressed vowels as well as high values of vowel articulation index. The words of children with DS, besides high pitch values of words and stressed vowels noticed for children with ASD, had maximum duration of words and stressed vowels, low values of the vowel articulation index of stressed vowels. In children with DS, more types of consonant phonemes were not formed compared to children with ASD and TD ones; in addition, children with DS used consonants that do not exist in the Russian language.