Congrats on your pregnancy! The thing about stretch marks is they are part of your genetics, If you are going to get them, you are going to get them! (I've been told this by countless doctors and nurses all through 3 pregnancies! ) There's no way to really prevent them. But you can help them fade. With my first pregnancy, I used Palmer's coco butter and didn't see a single stretch mark until 38 weeks, then they were popping up everywhere and itchy as hell! Whatever you do, DO NOT SCRATCH THEM! Scratching will make them 200% worse and even itchier! Every time they itch, lotion lotion lotion!
After my first baby was born I used Vaseline brand Coco Butter lotion and Bio Oil in the morning and before bed. 8 months later, they had faded so much you couldn't even tell they were there! :D But then, I was pregnant again and they came right back!
Now, 6 months after my third baby I'm just starting the Bio Oil again, and already noticing a difference. Bio Oil worked amazing for me, but I only used it after pregnancy because like I said if your genetics say you are going to get stretch marks when you are pregnant, there's no stopping them! So I don't think there's much of a point to use the Bio Oil while you are pregnant. Also I've heard that tanning helps, but I've never tried because I am very fair and burn easily, and as a mom of 3, I don't have time to go tanning! Good luck!