Coconut butter helps lighten them during and after pregnancy. I bought the cheap cocoa butter from Wal mart and its already helped.
Sorry but stretch marks never go away. They do lighten up over time but they'll always be there. Unfortunately there's really no way to prevent them, they're hereditary so some women get them and some don't but you should still moisturize every day. I rub 100% coco butter on my body and face every day. I always get compliments on how great my skin is but I do have some stretch marks.
They'll fade after baby but they likely won't disappear.