system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do babies breathe/live during the birthing process in between the fluid-filled womb and the air-filled world? How do they not suffocate during the pushing process?

My first child was born recently and as my wife was pushing I had this thought and almost went into panic mode. Thanks in advance for the help!
gpt
Before the first breath the foetal circulation is still intact, taking blood containing oxygen from the placenta to the foetus by the umbilical vein and returning blood without oxygen from the foetus to the placenta via the umbilical artery.

Only after the first breath that expands the infant's lungs, opening up the airways and the lung arteries and veins, can blood start to flow through the infant lungs to be where it picks up oxygen. Before this time the foetus is receiving oxygen from the placenta.

If the blood flow from the placenta is interrupted before first breath then the infant will become unwell due to lack of oxygen within minutes. Prolonged lack of oxygen causes brain damage and then death.

Note: Foetus (English English) = Fetus ('merican English)

Edit: to remove big words.