Stretch marks

Alena
I read different opinions about getting pregnancy stretch marks. Some say you will get it no matter what you do if you have it in your genes and some say you can prevent it by using oils, creams, etc. 
I was wondering if anybody has any idea when to expect seeing stretch marks appearing (on boobs, belly, hips) "if you have it in your genes" or not using any oils and have no way to escape them? 
Would they start showing during pregnancy as you gain weight or after you give birth?