I read about Dunstan Baby Language before ds was born, which claims that "across cultures and linguistic groups there are five sounds, each with a meaning, that are used by infants during the beginning of the language acquisition period. Between 0-3 months infants make what Dunstan calls sound reflexes.
"According to Dunstan, these sound reflexes are preemptive cries that occur before the infant breaks into what she calls the hysterical cry. Dunstan claims that these preemptive cries can indicate what the infant requires (e.g. food, comfort, sleep, etc.) and elevate to the hysterical cry if they are not answered. As the infant matures past 3 months in vocalization, the sound reflexes become replaced with more elaborate babbling.
According to Dunstan, the five universal words (or sound reflexes) used by infants are:
Neh
I'm hungry - An infant uses the sound reflex "Neh" to communicate their hunger. The sound is produced when the tongue is pushed up on the roof of the mouth and the sucking reflex is triggered.
Owh
I'm sleepy - An infant uses the sound reflex "Owh" to communicate that they are tired. The sound is produced much like an audible yawn.
Heh
I'm experiencing discomfort - An infant uses the sound reflex "Heh" to communicate stress, discomfort, or perhaps that they need a fresh diaper. The sound is produced by a response to a skin reflex, such as feeling sweat or itchiness in the bum.
Eairh
I have lower gas - An infant uses the sound reflex "Eairh" to communicate they have flatulence or an upset stomach. The sound is produced when trapped air from a belch that is unable to release and travels to the stomach where the muscles of the intestine tighten to force the air bubble out.
Eh
I need to burp - An infant uses the sound reflex "Eh" to communicate that they need to be burped. The sound is produced when a large bubble of trapped air is caught in the chest and is trying to release out of the mouth.
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I don't know about all of them, but my ds had a very distinct "neh" when he was hungry, and it helped a lot to be able to identify at least one of his cries. Now he's older (4 months) it's a lot less reliable, but he's still fairly easy to read because he chews his hands when he's hungry, yawns when he's tired, and doesn't seem to mind sitting in poo, so we have to sniff test him!