I am 38 +2 weeks and still have not had any... yet lol although I here of woman who have none throughout pregnancy but then get them after they deliver the baby. Imagine a shiny and smooth balloon after it deflates, it gets all rough and wrinkly. Thats probably what will happen with our bellies after the baby comes out lol!
You can't do anything to prevent them, but if you do get them creams and such will reduce the appearance of them. I got stretch marks on my boobs in the first trimester and now in the last trimester 36 weeks to be exact I'm getting them on my sides and belly.