You're not alone. Twenty percent of the Beauty Q&A letters we get are from readers like you. Just consider stretch marks one of the many curses of growing up! According to Beverly Hills dermatologist Sheri Feldman, M.D., stretch marks are common during adolescence (also during pregnancy and a period of sudden weight gain). They occur when your skin rapidly stretches -- like when you grow -- and your collagen bundles (which make up the major structural protein of the skin) tear. Scarring occurs and new collagen bundles form, leaving red or purplish streaks on your thighs, butt, or breasts. When they're brand-new like you said yours were, Dr. Feldman says that Retin-A may help in treating them. Unfortunately, once you've got them, they're yours for good.