What causes stretch marks?

I recently gained a lot of weight around christmas time. Around the end of January I stopped gaining anymore weight. Then in april strech marks started to form all over my stomach. They have know started forming on my arms, legs, armpit/breasts. Im 17...

and weigh 196 lbs. I've heard that heredity shows how likely you are to have stretch marks, but both my parents are obese and neither of them ever had strech marks. I didnt have any growth spurts around this time either. Is this a common way that people get strech marks?

weight gain

Often the stretch marks will only become apart a long time after weight gain. I didn't even notice I had stretch marks until my lovely friends decided that everyone MUST have stretch marks and decided to find all mine...thanks for that...Lol but point is, I hadn't put on any weight for like a year(if not much more) before that and had never noticed them before!
At first stretch marks may be very pale, then get darker, then start fading again and may eventually go white but be raised.
If you want to make them less noticable and prevent more try using shea butter, cocoa butter (both of which you can find in places like body shop) and BioOil. It probably won't get rid of them but it will make them fade. The sooner you use it the more effective it is.
Hope this helps x