Stretch marks happen when your body grows faster than your skin can keep up with. This causes the elastic fibers just under the surface of the skin to break, resulting in stretch marks. In an average pregnancy you gain between 25-30 pounds in 9 months. Growing that fast can leave you with stretch marks, especially on your belly and breasts, two areas that grow the most. Stretch marks can also show up on the thighs, buttocks, and upper arms. The marks often start out reddish or purple, but after pregnancy they gradually fade to white or gray.
About 90% of women will get them sometime after their sixth or seventh month of pregnancy, according to the American Academy of Dermatology. If your mother had stretch marks, then you're more likely to have them too, since genetics plays a role. Unfortunately, there’s no way to prevent stretch marks. It's always a good idea to keep your skin hydrated with a rich lotion or cream, especially if it makes your skin feel better, look smoother and more toned, and helps the itchiness that can come with your growing belly.
Story Adapt from: http://www.webmd.com/baby/features/stretch-marks