If you're pregnant right now the best thing to do is use lotion with Vitamin E. Vitamin E is really great is making the skin elastic enough and well hydrated to prevent stretchmarks from darkening. Or you can simply buy a small bottle of virgin olive oil and rub it on your belly twice a day especially on areas where stretchmarks are really dark. The result is a little slow but the good news is; it works!
You should also increase your fluid intake and vitamin C, E and A these are natural antioxidants.
Sometimes, since stretchmarks can appear like scars, you can also sure pure petroleum gel to lighten stretchmarks.
I hope this helps.