Stretchmarks start from the inside out, so hydrating with lots of water is important. Using creams and oils will help with the outer layers of skin. I think it's a combination of genetics AND prevention. My mother and both sisters got some, but I didn't. They also believed it was only genetics and didn't try to prevent them. If they had, who knows.
I kept very hydrated and used Lunar Belly Butter. It's a bit greasy but kept the skin very soft and smooth.