During the first few months of life, babies persistently attempt to bring their hands to their mouths but have not developed the strength or skills to keep them there. By 4 months, however, babies can put their hands in their mouths and keep them there (if they choose). They are now capable of clenching, shaking, and mouthing a rattle or of transferring a small object from one hand to the other. Because babies do not yet understand that some objects shouldn’t be in their mouths, adults need to keep the home environment free of potentially harmful items while still allowing their infants to explore.
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.