The Development of the Auditory System from Conception to Term

Author Disclosure Dr Lasky has disclosed that this review was supported in part by NICHD R01 HD042639-01A2. Ms Williams did not disclose any relationships relevant to this article. After completing this article, readers should be able to: 1. Explain when the fetus begins to respond to sound. 2. Describe the type of sounds to which fetuses and extremely preterm newborns initially respond. 3. Explain why the fetus is exposed to low-frequency sounds. 4. Describe the role of the fetal cochlea and the outer and middle ears in response to sound. 5. Compare and contrast the response to sounds in the fetus of the same postmenstrual age and preterm newborns. For the first half of pregnancy, the fetus is unresponsive to sound. By term, basic auditory capabilities are relatively mature. We review in this article the anatomic and functional developments during that remarkable interval between those two landmarks, which also is a time of increased risk for otologic insult. (1) We also comment on the environmental milieu during this critical period of development in normally developing fetuses and in preterm newborns. ### Definitions Sound is vibratory energy transmitted by propagating compressions and expansions of a stationary medium. The frequency of a sound (perceived as pitch) is defined by the number of compressions and expansions (cycles) per second or hertz (Hz). Human hearing ranges from 20 to 20,000 Hz, with greatest sensitivity at 1 to 4 kHz. The density and elasticity of the medium determine sound transmission through that medium. For example, the speed of sound in water (and amniotic fluid) is more than four times the speed of sound in air. Acoustic impedance is the complex ratio of the pressure induced by the sound source to the volume velocity of the vibrating medium. The impedance mismatch at boundaries between different media determines how much sound energy is transmitted …