Stretch marks are overwhelmingly a genetic thing... I am almost guaranteed to get them, because I have loads and loads of old ones just from puberty and a little adult weight gain (ironically, I get this from my dad, not my mom). Other people will slather themselves with oils and potions and swear this is what prevented stretch marks... But FTMP, those people never would have gotten them no matter what they did or didn't do. Lucky %^$@%$@#!!!
Not like moisturizing could hurt! And it certainly feels good. But there's been almost nothing that has shown any promise in clinical trials (and they've tested cocoa butter and all the usual suspects).
There's basically exactly one thing that actually has a suggestive Cochrane Review...
But I had Dr. Mom look at the research (my mom is a semi-retired research MD-- used to be with the FDA), and she was very meh on it... like MAYBE it MIGHT do SOME good. If I can convince myself the ingredients are unlikely to be absorbed/harmful to baby, I might try it, but other than that I'm resigning myself as much as possible.
One thing I do know, that has been proven, and has "worked" for me, is that if you put Retin-A (tretinoin) on fresh red stretch marks, they will fade much more quickly. Not be erased or anything, but at least fade to silver faster. I'm not sure if Retin-A is contraindicated for breastfeeding, but if not, I'll be all over that after baby is here.
AFAIK, BTW, most stretch marks don't happen until 35+ weeks. But I could be wrong on that!