[s1k3b8]

> I did it not for the posted reasons, but just beacuse it felt right to me.

It probably felt right because it's natural. Newborns pretty much stayed with their mothers 24/7 for all of human history. This is true for chimps ( our closest ancestors ) and even our common ancestor. It's only recently with the push to get women into the workforce where women were encouraged to separate from their newborns so that they could go back to work.

> As soon as they came out, I held them and sang and hummed the same tune to them. It immediately calmed them - with my first, she immediately relaxed and stopped crying whil the nurse took her vitals, measurements and pricked her heel.

The mother's voice also helps premature babies develop. The fetus probably has gotten accustomed to the mother's voice in the womb and associates it with comfort and safety. Perhaps it's an evolutionary relic since many newborn animals are drawn to their mother's voice and vice versa.

https://www.psychologytoday.com/intl/blog/shouldstorm/201909...

[animal531]

Human babies also learn the pre-constructs for the languages being spoken around them while still in the womb, so no reason they can't learn other repeated noises.