Ah, the million dollar question, right? What makes labor start? And to be honest? We actually don’t fully know. But we do know that there are a lot of hormones at play in this dance! It’s also important to point out that there isn’t just one thing that happens - it’s some sort of whole cascade of events. Let’s talk through three common or popular “theories.” 

1. The first theory is related to the secretion of oxytocin from the maternal pituitary gland. Oxytocin is a naturally occurring hormone that rises throughout pregnancy. It stimulates uterine contractions and also plays a role in bonding and breastfeeding. It’s believed that at some point, plasma oxytocin hits the level it needs to, triggers contractions, and when that coincides with a ripe cervix, labor ensues.
2. Alternatively, there is also a theory about the baby actually being the thing that triggers labor. There is research that shows that when baby’s lungs are fully mature, they release a certain protein that then triggers a prostaglandin wave in the maternal body that triggers labor to begin.
3. Lastly, there is a third theory related to the baby’s adrenal glands. Around 38-40 weeks of pregnancy, a baby’s adrenal glands begin functioning similarly to an adult. They can release cortisol, which interrupts the balance of progesterone and estrogen in the pregnant body, and labor begins.

So ultimately, we just don’t fully know! I do think most people would be proponents of the idea that giving the body time, and perhaps delaying induction until it is medically indicated, gives the maternal body and the baby the best chance to fully do all of these semi-mysterious things that it does.