By 4 months, babies begin listening for the individual sounds in words, noticing how vowels and consonants combine into syllables, words, and sentences. Their babbling often comprises repeated consonant sounds, like “ma-ma-ma” or “ba-ba-ba.” Also, their babbling begins to sound like unintelligible speech that uses the rhythms and characteristics of their native language. Babies often follow the lead of their caregivers, sometimes repeating a sound for a day or more before experimenting with a new sound. Although it may be more than a year before they start speaking intelligibly, many babies understand words before their first birthday.
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.