My Mum is covered in stretch marks so I was pretty resigned to the fact that I would get them. She put together for me an aromatherapy blend (focussing on the combination of base oils) which I used throughout my first pregnancy and I didn't get a single stretch mark.
Then after A was born I stopped using it, but also put on 2 stone, and now my hips and thighs are covered in them - so I obviously am prone to them!
This time i've been using it again and at 30wks I still don't have any (touch wood!!)