Some women don't get stretch marks because their skin reacts well to expansion. Others get stretch marks because genetically, their skin does not. If your mother has stretch marks from pregnancies or you have stretch marks from your growth period in your youth, then it's likely you'll get them from pregnancy no matter how much oil or lotion you rub every day on your belly. Thanks Mom.
Lotions do decrease itchiness that comes from rapid skin expansion, so go ahead and rub it on your belly if it makes you feel good.