[a3n]

But do babies say that from the very first, when they're playing with sounds as the article says? I'm guessing not, they learn it after awhile.

[hugh4]

Nah, it's more of a "ma" sound with a short "a".

Now what's interesting is that this sound is closer to how an American might pronounce a short "o" (I think of a southern belle fanning herself and saying "lawd it's haaaht") and how an Englishman might say a short "u". It's not exactly the same vowel but I can see how it develops.

Both versions of English think babies say "mama", but you can't shorten that to "mam" because you'd use a different "a" to pronounce that.