Congenital (something you're born with) hearing impairment occurs in about 0.3 percent of all children. For many children, their hearing loss is not detected until 14 months or older, when they fail to begin speaking. There are signs, however, to look for before children reach that age. Young infants should respond to loud sounds in a room. By 4 to 6 months, infants should turn toward a voice: If your baby is in her crib and you stand at the door where she can't see you and start talking, she should quiet down for a second, and then turn in the direction of the voice to try to localize it. Somewhere between 3 and 8 months, infants should imitate speech sounds; they don't make words but do imitate tone and, eventually, syllables.
When there is a concern about possible hearing impairment, the type of hearing test will depend on the age of the child. In older children who are cooperative (ages 3 and up), the standard headphone test is used: the child raises her hand or presses a button when she hears the sound. For younger children (12 months to 3 years), a variation of this is used: In a specially-designed room sounds are played from different corners and at varying decibel and frequency levels, and the child's response to them is noted. This test does not test each ear independently, however, and it wouldn't necessarily pick up a child who has a hearing impairment in just one ear. In infants (and uncooperative older children), hearing can be tested using auditory brainstem (area of the brain that helps us to hear) responses. In this more sophisticated test, electrodes are taped on the back of the head, similar to what happens for an EEG (brainwave test), and different levels of sound are transmitted to the baby's ear. The electrodes record the brainwave activity that occurs in response to the sound, and thus tells whether or not the sound was heard.