Your baby's external ears have been developed for some time but for him to hear, the internal ear structures also need to mature. In the middle ear, three bones-the malleus ("hammer"), incus ("anvil"), and stapes ("stirrup")-conduct sound into the inner ear. These bones are formed initially from soft cartilage and embedded within connective tissue. The bones begin to harden and the connective tissue gradually dissolves. This allows the ear drum to vibrate onto the hammer, which passes the movement on to the anvil, and then the stirrup. The vibrations are then passed to the cochlea, a cavity of the inner ear, where they are translated into nerve impulses to be sent to the brain.
At 22 weeks, your baby's inner ear has matured adequately for sound to be processed into neural signals to the brain. The first part of the cochlea to develop is responsible for receiving lower-sound frequencies. As your baby develops, he will gradually be able to recognize and respond to higher sound frequencies. Over the next three weeks, your baby's responsiveness to sounds will gradually increase. At first the responses are slow and sluggish, but by 25 weeks he will react to a range of sounds by moving around.
In addition to being responsible for your baby's hearing, the inner ear also controls his balance. Small fibers within three semicircular canals of the inner ear are able to sense acceleration in any direction, providing the sense of motion and balance. Floating in the amniotic fluid is similar to weightlessness and, although your baby is very active, he has no sense yet of moving up and down.