Stretch Marks can occur by rapid weight changes, like pregnancy. They are formed when the skin in certain areas is stretched and is not elastic enough to be able to morph with the body. Once they are formed they will be a streak in the skin that will eventually fade in color and be more of a glossy look. It is important to use lotions and other things to reduce or prevent stretch marks. The skin, especially during pregnancy, needs to be taken care of during the later stages of pregnancy when the skin will stretch. This site provides all the information you will need for stretch marks. It will go in depth on why they are formed, and what you should do to try and prevent or diminish the look of current stretch marks.