I think it depends on the bub! Emerson was tummy to back because he hated tummy time LOL. But I've heard of other bubs going back to stomach first (though not as common as tummy to back I must admit). I remember being awfully excited when it happened! I think he was very early - 5 or 6 weeks from memory!
Milo did back to tummy first, because he didn't really like tummy time.
I don't remember how early he did it, but I knw he didn't roll from front to back until 5 months, maybe a bit later than that.
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