Since it's genetics and creams won't really help, I've heard of either using a topical vitamin E or a cream with vitamin E as the main ingredient. It won't prevent stretch marks but will help them go away faster after the baby is born.
As pp have said they are genetic, you will get them our you won't. That being said, I found that when I was pregnant with my son that they really helped with the dryness and the itching that came with the skin stretching. I was very uncomfortable without it. I liked Palmer's stretch mark cream and burt's bees mama bee belly butter.
Save your money. They are genetic. It helps your skin to stay well hydrated so drink lots of water. And try to gain weight slowly. But if you're going to get them, you are going to get them.
The first two kids I did not get stretch marks, my third I did. I blame it on her being a girl and the first two a boy - its a joke.
I used cocoa butter and at night vaseline. It was a little messy but I wasn't looking for a scratching post as much when I rubbed vaseline in. . . and you have to rub it in pretty good.
Re: Stretch mark cream recs