Babies begin hearing well before they’re born. The earliest responses to sound appear around 23 weeks of pregnancy, and by 27 to 28 weeks, most fetuses react consistently to low-frequency sounds. From that point through birth and the first months of life, hearing rapidly sharpens into one of your baby’s most developed senses.
Hearing Begins in the Womb
The auditory system starts taking shape early in pregnancy, but it isn’t functional right away. Around 23 weeks of gestational age, the inner ear and the nerve pathways connecting it to the brain have developed enough for a fetus to first detect sound. At this stage, responses are inconsistent. By 24 weeks, fetuses show a startle response to vibrations and loud stimuli.
Things change quickly over the next few weeks. By 27 weeks, 96% of fetuses respond to low-pitched tones (250 Hz and 500 Hz), which fall in the range of a deep male speaking voice. Higher-pitched sounds take longer to register. Responses to 1,000 Hz tones don’t appear until around 33 weeks, and 3,000 Hz tones (closer to a high-pitched whistle) aren’t reliably detected until 35 weeks. As the fetus matures, the volume needed to trigger a response drops by 20 to 30 decibels, meaning the auditory system is becoming dramatically more sensitive week by week.
What a Fetus Actually Hears
Sound reaching a fetus is not the same as sound in open air. External noises must pass through the mother’s abdominal wall, uterine tissue, and amniotic fluid before arriving at the baby’s head. Low-frequency sounds (below about 1,000 Hz) travel through remarkably well, losing only about 6 decibels of volume. That’s barely noticeable. Higher frequencies, though, get muffled significantly, losing 20 to 30 decibels or more by the time they pass through tissue, and then another 40 to 50 decibels passing through the fetal skull bones.
The practical result: your baby in the womb hears low-pitched sounds quite clearly but probably cannot detect much above 500 Hz at comfortable listening volumes. Voices come through as deep, rhythmic patterns rather than crisp words. Music sounds bass-heavy and muted. The constant backdrop is the mother’s heartbeat, blood flow, and digestive sounds, all of which are low-frequency and surprisingly loud inside the uterus.
Your Voice Is Already Familiar
By the third trimester, a fetus is not just hearing sounds but learning from them. Multiple studies have measured fetal heart rate changes in response to the mother’s voice compared to a stranger’s voice. While the direction of the heart rate shift varies across studies (some fetuses speed up, others slow down), the consistent finding is that fetuses respond differently to their mother’s voice than to an unfamiliar one. Newborns confirm this: they show a clear orienting response to recordings of their mother’s voice, becoming calmer and moving less compared to hearing a stranger speak.
This recognition extends to specific sounds. In a study at the University of Helsinki, mothers played a recording of “Twinkle Twinkle Little Star” five times per week starting at 29 weeks of pregnancy. After birth, the babies’ brain activity was measured while hearing the melody. Infants who had been exposed in the womb showed significantly stronger neural responses to the familiar tune than babies who hadn’t heard it before, and the more often they’d heard the melody prenatally, the stronger the response. Remarkably, this difference was still detectable at four months of age. The fetal brain doesn’t just passively receive sound. It forms lasting memories.
Hearing at Birth and the First Months
A newborn’s hearing is functional from the moment of birth, though it continues to mature. In the first days of life, most hospitals screen your baby’s hearing using one of two quick, painless tests. One measures the electrical activity in the auditory nerve in response to a soft clicking sound played through a small earpiece. The other detects faint sounds produced by a healthy inner ear in response to tones. Both are typically done within the first few days, often while the baby sleeps.
During the first three months, you can expect your baby to:
- React to loud sounds with a startle
- Calm down or smile when spoken to
- Recognize your voice and settle if crying
- Pause or change sucking patterns in response to a new sound
Between four and six months, hearing-related behavior becomes more sophisticated. Your baby will start following sounds with their eyes, respond to shifts in your tone of voice, notice toys that make noise, and pay attention to music. Babbling picks up during this period too, with speech-like sounds emerging that include consonants like “p,” “b,” and “m.” This babbling is directly tied to hearing. Babies are listening to the sounds around them and beginning to experiment with producing their own.
Protecting Fetal Hearing
Because the maternal abdomen does surprisingly little to block low-frequency sound, prolonged loud noise exposure during pregnancy is a real concern. The CDC recommends that pregnant women avoid environments louder than 115 decibels, which is roughly the level of a loud rock concert or a chainsaw at close range, even while wearing hearing protection. Earplugs or earmuffs protect the mother’s ears but do nothing for the fetus, since sound reaches the baby through the body rather than through the ear canal.
Low-frequency noise deserves particular caution. Sounds you feel as a rumble or vibration pass through tissue most efficiently and reach the fetus with very little loss. If you work in a loud environment with heavy machinery, engines, or sustained low-frequency noise, it’s worth discussing your exposure level with your healthcare provider. Brief encounters with loud sounds, like a fire truck passing or a single loud event, are not considered a risk. The concern is repeated, prolonged exposure over weeks or months during the third trimester, when the auditory system is most actively developing and most sensitive.