Hearing begins in the last trimester

The development of hearing in a future baby starts gradually during pregnancy. The auditory system begins forming in the earliest weeks, but the fetus does not perceive sounds immediately. Hearing begins in the last trimester, from 28 or 30 weeks of amenorrhea, according to Samuel Salama. His hearing continues to develop until birth.

Amniotic fluid affects sound perception

In the amniotic fluid, the fetus hears very diminished and variable sounds, mainly internal noises of the mother, such as the heartbeat and digestive noises. At first, the sounds are difficult to distinguish, and the baby mainly perceives vibrations and muffled sounds, especially those produced by the mother's body. Over the weeks, his auditory system becomes more mature and allows him to better respond to the sounds around him.

First trimester ear development

In the first trimester, the first structures of the ear form. From the first weeks of pregnancy, the inner ear, middle ear, and outer ear develop gradually. At this stage, the baby cannot yet hear like a newborn. His auditory system is under construction and not mature enough to process sounds.

Second trimester sound perception

In the second trimester, the baby begins to perceive sounds. From the middle of the pregnancy, generally around 20 to 24 weeks, the auditory system gradually becomes functional. The baby can then begin to perceive certain sounds, notably the noises coming from the mother's body: the heartbeat, blood circulation, or digestive noises. External sounds can also reach him, but they are attenuated by the maternal tissues and the amniotic fluid. The mother's voice is particularly interesting, because it is transmitted both from inside the body and through vibrations.

Third trimester hearing maturation

In the third trimester, hearing continues to mature. The baby becomes more receptive to certain sounds and can react to sounds that are sufficiently perceptible. He may notably show changes in movement or heart rate in response to a sound stimulus. These reactions do not necessarily mean that he understands what he hears, but they indicate his increasing sensitivity to sounds. As the birth approaches, he is also able to become familiar with certain voices and certain sound rhythms.

Postnatal sound recognition

As for being able to analyze the sounds of voices, it is not certain and there is no tangible evidence. However, postnatally, after birth, it is noticed that the baby recognizes the voice of his mother. The baby can also perceive certain sounds from the environment, such as music, conversations, or household noises. However, these sounds do not reach him with the same clarity as someone located outside the uterus. The frequencies and intensities are modified by the maternal tissues and the amniotic fluid. It is therefore preferable to favor a usual and pleasant sound environment rather than trying to stimulate the baby's hearing with particularly loud sounds.

Variability in fetal responses

Not all babies react the same way to sound stimuli, and their movements are not always easily associated with a specific noise. A baby can react to a sound with a movement, a change in activity, or a variation in heart rate, without these reactions being systematic. The absence of a visible reaction to a voice or music does not necessarily mean that he does not hear. His position, his state of sleep, and the stage of his pregnancy can notably influence his reactions. An unusual decrease in the baby's movements should be taken seriously. If the baby does not move for several hours, it should not be hesitated to contact the midwife or gynecologist to ensure the child is well.