I asked my midwife because I was feeling guilty that I hadn't been doing any - she said not to worry about it and that your skin is either naturally stretchy or it isn't.

I had two tiny stitches after 2.5 hours of pushing with a biggish baby and forceps so I reckon my skin is pretty stretchy! I didn't get any stretchmarks either and I think, from memory, she said that's related.