Question: do you now know why women who are pregnant go in to labour i haveallways wondered why consraction start is it because the babies get to big?

Gemma Sharp answered on 16 Jun 2011:

This is a brilliant question.

I’m going to go into quite a bit of detail, because I like talking about it! :p
It’s complicated, believe me I know! If you don’t understand anything, just comment and I’ll get back to you…

We know why pregnant women go into labour (to get the baby out!) and we kind of know why it happens at 9 months (because the baby is well-developed enough to live outside the womb, but not so big that it won’t be able to fit through the mum’s pelvis), but we don’t know what tells the contractions to start.

There are loads of theories, but here are the main ones:

* Stretch – as the baby gets bigger it starts to stretch the muscle layer of the uterus (womb) and that causes the muscle to start contracting

* Hormones from the placenta/foetus – towards the end of pregnancy the placenta starts making more of a hormone called CRH which causes the foetus to produce more steroid hormones. The steroids stimulate the foetus’s organs to develop more, especially the lungs. As the lungs develop they start producing more of a hormone called ‘surfactant’, which ends up in the amniotic fluid the foetus is living in. From there, the surfactant triggers the muscle layer of the uterus to start contracting.

*Hormones from the mother – During pregnancy there are high levels of a hormone called progesterone. Progesterone keeps the muscle layer of the uterus relaxed. Towards the end of pregnancy, the uterus isn’t as responsive to progesterone, it kind of starts ignoring the high concentrations, so progesterone has less of an effect and that might make the uterus start contracting.

*Inflammatory response – Just before labour starts, loads of white blood cells turn up in the uterus. The same thing happens if you cut yourself; loads of white blood cells will turn up at the site of the cut to fight off any bacteria that might try to get in. This is called an ‘inflammatory response’, and there’s a lot of evidence that the inflammatory response can trigger the uterus to start contracting.

Right, so all of those theories sound pretty plausible. That’s because they are. They probably ALL happen, but we don’t know which one is most important, or what sort of drug would stop them happening if it looked like a woman was going to go into labour too soon.

That’s what I’m trying to find out! I’m focussing on the last theory (inflammation) because I *think* that might be the most important one… 🙂