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.
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.