Stretch marks are a normal part of puberty for most girls and guys. When a person grows or gains weight really quickly (like during puberty), that person may get fine lines on the body called stretch marks.
Stretch marks happen when the skin is pulled by rapid growth or stretching. Although the skin is usually fairly elastic, when it's overstretched, the normal production of collagen (the major protein that makes up the connective tissue in your skin) is disrupted. As a result, scars called stretch marks may form.
If you're noticing stretch marks on your body, you're not alone. Most girls and women have stretch marks, which tend to show up on the breasts, thighs, hips, and butt. Many women get them during pregnancy. And while they're more common in girls, guys can get stretch marks, too.

People who are obese often have stretch marks. Bodybuilders are prone to getting stretch marks because of the rapid body changes that bodybuilding can produce. Stretch marks also may occur if a person uses steroid-containing skin creams or ointments (such as hydrocortisone) for more than a few weeks, or has to take high doses of oral corticosteroids for months or longer.

At first, stretch marks may show up as reddish or purplish lines that may appear indented and have a different texture from the surrounding skin. Fortunately, stretch marks often turn lighter and almost disappear over time.
But the fact that stretch marks usually fade and become less noticeable over time can be little consolation if you plan to spend most of your summer in a bathing suit.