Stuttering is a difference in speech pattern involving disruptions, or “disfluencies,” in a person’s speech. 

People who stutter may experience repetitions (D-d-d-dog), prolongations (Mmmmmmilk), or blocks (an absence of sound), or can experience some combination of these sounds. 

No one's speech is 100% fluent all the time. Some disfluencies or disruptions in your child's speech are normal. 

Source: https://www.westutter.org/post/what-is-stuttering