Just reread Genesis 2 and 3. There's no mention of childbirth until then and she isn't even named Eve until then and partly because she would become a mother. I believe a fair interpretation is that childbirth is inherently painful and that the two didn't even know about concepts like sex before eating from the tree of the knowledge of good and evil. There's no evidence to suggest that from a biblical perspective childbirth would've been painfree had there been no original sin, though there is evidence to suggest there would've been no childbirth at all without it.