system
Given the following post, respond as if you're commenting on Reddit:
human
How and when does a newborn begin to breath air?

How exactly is oxygen delivered to the baby in the uterus, and when does that process stop and the breathing start? What part of birthing the baby triggers that? Also, in the uterus, isn't the baby surrounded by fluid? Does that fluid need to be coughed up on birth?
gpt
In utero, oxygen is delivered to the fetus directly from the mother's blood via the umbilical cord. The lungs are developed by about week 38, but are not used until after birth. You'll notice after birth, the doctor or nurse will suck out some fluid from the baby's nose - that's the amniotic fluid being removed. Also, you'll notice that when babies on TV are born they're smacked to make them cry - aka, breathe. That cry is the first breath babies take. In the real world, though, we haven't been practicing this since the 50's - these days if a baby hasn't started breathing by the time it's out of the vagina, some mild physical stimulation will be applied (it will be rubbed, its feet flicked, etc.)

**Edit**: Fixed some vagueness in stimulating newborns to breathe.

Interestingly along these lines, there's a sort of movement of giving birth to babies underwater. The theory is that going from amniotic fluid to water is less shocking for the baby than amniotic fluid to air, and as long as the baby's umbilical cord is still attached, the baby will still be able to receive oxygen. Interestingly enough, babies also have the reflex to not take breaths in water even directly after birth.