Try not to stress over stretch marks, because there is nothing you can do to prevent them. Either you'll get them or you won't, and the decision-making factor has more to do with genetics than any fancy creams or balms you slather over your expanding body. Case in point: I gained 41 pounds during pregnancy and didn't get them — despite the fact that my boobs alone went from a 36B to a 38E. Turns out my mom didn't get them either. But rest assured I did have other body-altering repercussions, and to that I simply had to say, that's life. However, itching that accompanies stretching skin is a symptom you can do something about. Specifically:
Keep the skin moisturized as best you can. Go for unscented stuff; it's less likely to irritate.
Take colloidal oatmeal baths
Colloidal oatmeal is extremely fine ground whole oats, which is known to relieve itches and soothe skin. You can make your own if you grind the heck out of oats with a coffee grinder (essentially to powder form) and drop a couple of cups worth into a warm bath. But you can just as easily buy Aveeno Oatmeal Bath from your local drugstore. For optimum relief, make sure the bath is warm, not hot, and gently pat yourself dry afterwards.
Avoid hot showers
Yes, they're great, but they can dry out your skin, which may contribute to overall itchiness.
Wear comfy clothing
Loose-fitting cotton gear tends to work best.