it's all in your genes. good stretchy skin=fewer stretch marks. i don't think any of the creams/lotions/potions do a thing.
i didn't get them until around 37 weeks last time but i'm thinking i'm not going to be so luck this time around.
gaining a lot of weight in a short amount of time adds to the stress on your skin but that's not really worth worrying about when you're pregnant and hungry :)
good luck!