When a woman goes into labour, is it her body that decides the baby is ready, or does the foetus send some sort of signal to the mother’s body to say it’s ready to come out?

Question

As above. Also, how would the answer to this title question explain babies that are born premature, or babies that are born so late that the labour has to be induced?

Thank you in advance! I have wondered this for a while!

In: 161

Answer 1

In addition to other answers, there are several emergency eject buttons, as it were.

Most pregnant women won’t break their bag of water like you see in movies, but if it does happen due to either membrane sweeping (a way to induce labor,) trauma, or just pure happenstance, two things result: the fluid from inside the bag will come into contact with the mother and the bag will begin to deflate. Both of these will trigger the start of active labor in the mother. Her cervix will begin to dilate and her uterus will start ushering the baby into position if it isn’t already there.

This is really useful for something like preventing the baby from getting too big before birth because a will hopefully break the bag before getting too big to be born (obviously not always the case, but you can see the evolutionary benefit to having a semi-fragile bag.) Our embryonic development is limited by our brain growth, because the skull and the shoulders are the biggest diameters when we’re born. Newborn heads look straight up like [xenomorph skulls](https://bloody-disgusting.com/wp-content/uploads/2020/12/125689_Special-Effects-Mechanical-Alien-Creature-Head_7.jpg) because they have to be squeezed to fit through the birth canal.