system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What triggers or causes labor? Like how does the fetus decide when it’s time to come out?
gpt
Simply, chemicals. 

There are a few hypotheses:

* Hormones from the mother - oxytocin production increases towards the end of pregnancy and triggers contractions (this is actually what the hospital gives you when they induce labour and want contractions to begin).
* A protein produced by the baby's lungs has been postulated to trigger labour when the lungs have sufficiently developed to survive outside. This may make sense as the lungs are near the last organs to develop. 
* Cortisol production in the baby near the end causes a hormonal imbalance of progesterone and estrogen, which has been hypothesised to induce labour.