We know that the inner ear completes its formation in the 16th week of gestation, but the functionality of the auditory system in the medical literature has so far only been confirmed from the 26th week onwards. This research work shows for the first time that the foetus can hear from the 16th week.

The foetus receives sounds from inside its mother's body, such as her heartbeat, breathing and bowel movements. It also picks up sounds from what its mother is doing, when she is talking or walking in her heels, as well as hearing noises from outside.

The foetus is very protected from noise. The fact that it lives in a soundproof environment means that sounds reach it distorted, as has been confirmed by research on sheep using intrauterine microphones. According to these studies, most sounds reach the foetus as a whisper (about 30 decibels), while the mother's voice in normal conversation (60 decibels) hardly reaches him at all (24 decibels).

Moreover, as most of the sounds are very repetitive, he gets used to them and does not react to them. They do not prevent him from sleeping.

Thus, we can say that the sound environment of the womb is like the background noise of a forest.

It is only possible through a single route, the vaginal route.

The vagina is a closed space, so sound is not dispersed into the environment. In addition, there are fewer layers of soft tissue separating the baby from the sound source, only the vaginal and uterine walls. By placing a speaker in the vagina, the barrier of the abdominal wall is removed and the baby can hear the sounds almost as loudly and clearly as they are emitted.

Prior to the ultrasound, the pregnant patient was fitted with the vaginal device designed for the study, which emits an average sound intensity of 54 decibels (the equivalent of a low-pitched conversation or background music).

87% of the foetuses reacted with non-specific movements of the head and limbs, and specific movements of the mouth and tongue, which ceased when the music stopped. In addition, with vaginal music, about 50% of the foetuses reacted with a very striking movement, opening their jaws wide and sticking out their tongues as far as possible.

Headphones were placed on the pregnant woman's abdomen, emitting music with an average sound intensity of 98.6 decibels (equivalent to an ambulance siren or music in a discotheque).

During this part of the study, no changes were observed in the facial expressions of the foetuses.

As it is a response and not a reflex, the foetal reaction depends on multiple factors and is therefore different from moment to moment. It varies according to the neural activity in the brainstem at that moment, so the response may depend on the phase of sleep the foetus is in or on blood glucose levels.

For example, when we sing to a baby, it also reacts differently depending on whether it is hungry, thirsty or sleepy.