I posted the same thing a few days ago. After doing some research on the web it sounds like a hormonal thing.

"Both hormones are basically designed to relax your uterus so you don't go into labour early, but they also relax all of the joints and ligaments around your pelvis so that when you do go into labour your baby will pass more easily through the pelvis"

I also read somewhere the pain can continue after birth and you may need physio. I vaguely rememember something about the space stretching from 1-2 cm to 3-4cm aswell or something like that.