After 2 months, some infants begin repeating a few of the exaggerated vowel sounds they hear when their caregivers speak “Parentese” with them. These sounds are usually vowel sounds (ooh, oh, or aah sounds, for example). As infants imitate sounds in response to their caregivers, they also begin to learn some basic rules of conversation like taking turns.
Shelov, S. P. (Editor-in-Chief). (2004). Caring for Your Baby and Young Child: Birth to Age 5. The American Association of Pediatrics. Revised edition. New York: Bantam Books.