The Origin of Hearing

What a joy it is to hear your baby laugh out loud at a funny sound or to see them wiggle to a tune they like. These delightful spontaneous reactions that result from healthy hearing are the product of an extensive development that began long before these little treasures arrived among us. Let's go back to the first weeks of gestation to understand how far they've come!

The First Weeks

The auditory system begins to develop as early as the fifth week of pregnancy. Two small spots appear on each side of the fetus’s head. These slowly turn into inner ears.

The first turn of the cochlea is formed at 7 weeks, and by 10 weeks, two-anda-half turns are complete. Cilia then appear inside the cochlea and allow sound stimulation to be coded into an electrical nerve impulse, which is then carried to the brain by the auditory nerve fibers.

The cochlea reaches its adult size in the fifth month of gestation. At this point, it is considered functional, since fetal responses to acoustic stimulation can be recorded.

Baby Is Listening!

Of the five senses that develop during gestation, hearing is the fetus’s sharpest sense.

Hearing begins between the 25th and 27th week. In fact, at this stage, the fetus hears its mother’s digestive sounds and heartbeat, which corresponds to a sound level of about 30 decibels.

Soon after, the fetus begins to perceive certain sounds from the outside world, such as voices and the sound of music (mainly low tones and vibrations). These sounds may elicit a reaction: their heart rate increases, their breathing changes slightly, and they make movements when certain sounds reach them. They may even startle a bit at sudden, high-intensity sounds.

From the 32nd to 35th week of pregnancy, the middle ear cavity, the external auditory canal, and the outer part of the ear are completely formed.

Finally Here and Almost All Ears!

Studies1 have shown that infants can "remember" certain sounds that they frequently heard in the womb. The baby can recognize repeatedly played melodies after birth. So, it would not be surprising if a lullaby regularly played or sung during pregnancy had a particularly soothing effect. This auditory memory also allows them to recognize their mother's voice from the first days of life.

Of course, the infant will perceive sounds much better once out of the womb. What’s more, they grasp many more sounds than you might think. The sounds they are able to produce are not representative of what they actually hear.

As years go by, the more the child picks up surrounding sounds, the more they are able to understand what is being said. From that point on, the possibilities are simply endless!

Newborn Hearing Screening

Studies show that 4 to 6 out of every 1000 babies are born with hearing loss2, and permanent hearing loss in both ears affects about 1 to 3 out of every 1000 newborns2. Newborn screening tests are therefore recommended for early detection of hearing problems in newborns (between 0 and 3 months).

The test consists of inserting a small probe into the baby's ear. The device emits sounds and records the auditory system’s response. If the auditory structures are working properly, they produce an echo. Test results are available as soon as the test is completed. This service is offered at select Lobe clinics (look for the symbol in the list of addresses on pages 30 and 31).

References:

1. Chelli, D., & Chanoufi, B. (2008, 7 juin). Audition fœtale. Mythe ou réalité ? ScienceDirect. https://www.sciencedirect.com/science/article/ abs/pii/S0368231508002275?via=ihub

2. Programme québécois de dépistage de la surdité chez les nouveau-nés. (s. d.). Accueil - CIUSSS de l'Estrie - CHUS. https://www.santeestrie.qc.ca/ clients/SanteEstrie/soins-services/Themes/ Grossesse-accouchement/depliant_ surdite.pdf