I didn't scratch any of the places I have stretch marks at, and they're there. Scratching doesn't matter, but once you have stretch marks it's bad to scratch them.
Don't scratch your stomach. I never did and never got stretch marks and drink lots of water, and keep your skin moisturized
You can't "prevent" stretch marks. They're hereditary, you can drink a lot of water because that makes your skin have more elasticity, and use a lot of lotion on your stomach, butt, hips, thighs, and lower back. Nothing can really prevent them though.
Your dr should have told you what foods to stay away from.
Sushi (unless it's cooked), lunch meat (unless you heat it up first), certain types of fish and tuna because they contain too much mercury, lower your caffeine intake to about 200mg a day, go ahead and invest in leggings and a bigger bra lol.