Why do we get stretch marks?

Stretch marks, or striae, appear in the dermis, the middle layer of skin, when it is stretched faster than it can adapt. Common causes include growth spurts in puberty, pregnancy, rapid weight gain and fast muscle growth. Hormones matter too: cortisol weakens elastic fibres, which is why long-term steroid use can cause them. They often begin red or purple and fade over time to silvery or pale lines. They are extremely common and harmless; treatments such as creams, laser and microneedling may improve appearance but rarely remove them completely.