system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how do babies know to start breathing as soon as they're born?
gpt
Actually, they start "breathing" in utero. However, the amniotic fluid is much easier on their system. During birth, the babe "holds its breath" through the birth canal but can often still breath via the umbilical cord. Otherwise any birth lasting longer than 3 minutes would be terminal.

After birth, they spit up the amniotic fluid & take their first breaths of gaseous air. Diaphragm motion is automatic. The air stings their lungs. Some think it's just because the lungs are new while other say it's because of the temperature difference as room temp is much colder than body temp. Thus they usually cry when born & start breathing. Within a few minutes, the fluid is all expelled & the air no longer stings.

Deep sea divers can use a breathing fluid based on amniotic fluid for oxygen exchange. It warms inside & feels "heavy but comforting" once the initial fear of drowning is past. Surfacing & emptying for regular air breathing has similar discomforts & may be similar to a newborn's experience.