Babies begin responding to sound well before birth. The first measurable responses to sound have been observed as early as 19 weeks of gestational age, when a fetus reacts to low-frequency tones. From that point, hearing develops rapidly through the rest of pregnancy and continues to sharpen throughout the first year of life.
Hearing Begins in the Womb
The fetal auditory system starts picking up sounds during the second trimester. In studies tracking fetal responses to pure tones at different frequencies, the earliest behavioral response appeared at 19 weeks of gestational age, specifically to a 500 Hz tone (roughly the pitch of a speaking voice). Between 19 and 35 weeks, fetuses gradually become responsive to a wider range of frequencies as the auditory system matures.
By the third trimester, a fetus is hearing enough to begin learning. The mother’s voice is the dominant sound source, transmitted through the body as vibration and muffled audio. This repeated exposure has a measurable effect: full-term newborns show distinct brain activity when they hear their mother’s voice compared to a stranger’s voice. Premature infants tested at the same developmental age don’t show this same distinction, suggesting that the final weeks of pregnancy play a key role in building voice recognition.
What Newborns Can Hear
From the moment of birth, babies respond to sound through reflexes. The startle reflex causes a newborn to pull their arms and legs inward after a loud noise. The Moro reflex, a related response, involves the arms flinging outward and then drawing back in. These are involuntary reactions, but they’re reliable signs that the auditory system is working.
In the first three months, you can expect a baby to react to loud sounds, calm down or smile when spoken to, and recognize a familiar voice. Many parents notice that a crying baby will quiet when they hear a parent speak. During feeding, babies may start or stop sucking in response to a nearby sound. These are all normal indicators of healthy hearing in the earliest weeks.
Turning Toward Sound: 4 to 6 Months
Around four months, babies shift from simply reacting to sounds to actively seeking them out. This is when you’ll notice your baby turning to look toward a new sound, following noises with their eyes, and paying attention to music or toys that make sounds. They also become more sensitive to emotional tone and will respond to changes in your voice, such as the difference between a playful and a stern pitch.
Babbling begins in earnest during this window. Babies start producing speech-like sounds using a variety of consonants, particularly p, b, and m sounds. They babble when excited, when unhappy, and when playing alone. Laughing also typically appears during this period. All of this vocalization is directly tied to hearing: babies babble because they’re listening to the speech around them and experimenting with reproducing it.
Understanding Words: 7 to 12 Months
The second half of the first year is when hearing starts translating into comprehension. Between 7 and 12 months, babies begin turning and looking in the direction of sounds with purpose, not just reflexively. They listen when spoken to and start to understand words for common objects like “cup,” “shoe,” or “juice.” They respond to simple requests like “come here.”
Babbling becomes more complex, with longer and shorter strings of sounds (“tata,” “upup,” “bibibi”). Babies at this age also babble strategically to get and keep your attention, use gestures like waving or holding up their arms, and imitate different speech sounds they hear. By their first birthday, most children have one or two recognizable words, often “mama,” “dada,” “hi,” or “dog.”
Newborn Hearing Screenings
All babies should have their hearing screened before leaving the hospital, or by one month of age at the latest. Two types of tests are used. One measures whether parts of the inner ear respond to sound. The other measures how the auditory nerve and brainstem carry sound signals to the brain. Both are painless and typically done while the baby is sleeping. These screenings catch significant hearing loss early, which matters because untreated hearing problems can delay speech and language development.
Signs of Possible Hearing Problems
Because hearing milestones follow a fairly predictable timeline, missed milestones can be an early signal that something needs attention. In the first three months, a baby who doesn’t react to loud sounds, doesn’t calm to a familiar voice, or doesn’t change their sucking pattern in response to sound may need further evaluation.
Between 4 and 6 months, watch for a baby who doesn’t turn toward new sounds, doesn’t notice toys that make noise, doesn’t respond to changes in your tone of voice, or isn’t babbling with a variety of sounds. From 7 months to a year, key concerns include not turning toward the source of sounds, not responding to simple words or requests, and not imitating speech sounds.
No single missed milestone is cause for alarm on its own, since children develop at slightly different rates. But if several items are missing within an age range, or if your instinct tells you something is off, that’s worth raising with your child’s doctor. Early identification of hearing loss leads to significantly better outcomes for speech and language development.
Protecting Your Baby’s Hearing
Infant ears are more vulnerable to noise damage than adult ears. Sound at or above 85 decibels can cause hearing loss, and very loud sounds can do damage in less than 15 minutes. For context, 85 decibels is roughly the level of heavy city traffic or a loud restaurant. A rock concert or fireworks display can easily exceed 100 decibels.
If you’re using a white noise machine in your baby’s nursery, keep it at a low volume and place it away from the crib rather than right next to their head. At loud events like sporting games, concerts, or fireworks shows, infant-sized noise-reducing earmuffs provide a practical layer of protection. The general rule is simple: if you have to raise your voice to be heard over a sound, it’s loud enough to pose a risk to small ears.