Pardon my ignorance. How does it get passed down in the case of regular births?
Natural birth is messy. Infants get covered in all sorts of slime, some of which occurs at "first light", right outside the body.
That bears repeating. One of the grossest things I've ever witnessed (which wasn't something dead) was my son immediately after he was born. I was impressed.
The bacterial transfer happens after the placenta has broken and the baby is being pushed out of the vagina. The vaginal opening and anus are inches away. The woman is bearing down to push out the baby and there can be discharges from the anus. Even if that doesn't happen, the area around the anus isn't entirely antiseptic.