You won't be able to stop it spreading, and not much has helped my relief. I had it for two months before I had my daughter, and it was BAD (all over my belly, arms, hands, and feet), and I have it again this time, but luckily, even though it broke out early, it's just around my belly and hasn't shown signs of spreading yet...
The only good thing is that it does go away pretty much immediately after birth! Hope you can find some relief. I've been using hydrocortisone cream when it gets really bad, otherwise I just try not to scratch :(