There are several things you can do that will help you prevent stretch marks in pregnancy.
1) drink lots of water daily water will keep you skin hydrated and purified.
2) watch your diet try not to gain too much weight a typical pregnancy is between 25-30 pounds
3) exercise do little work outs everyday to increase blood circulation
4) use a stretch mark cream made for prevention see best stretch mark cream during pregnancy
5) Don't scratch your belly!