I wouldn't worry about it until you are around your third trimester. You wont start getting stretch marks until your skin has stretched to it's max capacity and the only way for it to stretch more is to create more skin area.. aka stretch marks. I'm 7 1/2 months and no stretch marks yet. I didn't start using creme until 6+ months.
Just make sure when you do, you put it everywhere.. belly and boobs, maybe even hips. I haven't got stretch marks on my belly, but I do on my boobs. So be careful of that.
