I am still new to this site so will not be offended if you do not take my advice... However, I am only 13 weeks pregnant and have already started using Bio Oil.
My friend just gave birth and she was tiny so her skin had to stretch! she used it from the start and got no stretch marks at all. I know a couple of other girls with the same successful result..
You can get it from the chemist, I wet my hands and use just a few drops after a shower- smells nice too.
(not sure if this is something I am allowed to post or if it is classed as advertising- I don't have shares or anything honest!)
Bev