Im using bio oil. I'm now just over 35 weeks and no stretch marks. I didn't think to use it on my bum and I have quite a few there, but they are hidden by underwear I'm not too worried about those.
I've been using mama bee's belly butter. I think it works prettt well because I've always had some stretch marks so I know I get them easily but I haven't gotten any new ones yet.
I Use it once a day. But I'm not to sure whether it works or not..
I've heard bio oil works good, I use palmers cocoa butter for stretch marks, and I've only gotten one stretch mark but that's because a tiny scar I had on my tummy that got stretched out. So I love it :)