Lots and lots of times during pregnancy, I was reminded to keep moving, to keep on exercising. Everyone told me that - mothers I knew, the midwives and that bible I grew to dislike quite a lot 'What to Expect when You're Expecting'. What nobody told me was HOW and WHY it was so important. I did find out in the end - birth is a big physical deal. It takes stamina and strength. So all the advice I got was totally correct but given to me in the wrong way.
It wasn't only because you need to keep your weight down (you do, still do that), it wasn't just because it's healthier for the baby (it is) and it wasn't even because it will keep the aches and pains at bay in the final weeks (some days I could only walk if I'd done some gentle yoga or been for a swim early in the morning). It's because to give birth with as few as interventions as possible, you need to be able to give it all you have.
It's almost like planning a long distance run. You need to be fit, well fuelled but not too full and as well rested as possible.
I didn't know it until I felt it, but once labour starts, every ounce of energy you have will be diverted to your uterus. And once it has grown to a height of 40cm, it's a pretty big muscle. So if I'd have known that I needed to keep fit to improve my chances of a more straightforward birth, I would have applied myself with a bit more gusto.
The advice then: if you have an exercise routine, stick with it for as long as possible, even if in a gentler, modified form. If you don't have one, take up pregnancy yoga, swim if you can or get walking. You'll be so grateful you did.