By 4 months, babies begin listening for the individual sounds in words, noticing the way vowels and consonants combine into syllables, words, and sentences. Their babbling often is made up of 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 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.