Does anybody know? I've just realised I have no idea. I kind of assumed it would be a bit like the inside of your cheeks, but during pregnancy the uterus has the capability to expand to something like 1000x its volume and then more or less return to its original size. Most other skin-like structures on the body would give up and go extremely saggy after much less stretching than this - just look at tv programmes about weight loss. It must be amazing stuff to be able to cope with such drastic size changes.