Stretch marks are long, narrow streaks, stripes or lines that develop on the skin and which differ in hue from the surrounding skin. They are the result of a sudden stretching of the skin and are extremely common. Anyone can develop stretch marks, although they tend to affect more women than men.
Stretch marks can be visible on body parts including the tummy, thighs, hips, breasts, upper arms, and lower back. The marks form in the middle layer of the skin; when there is a constant stretch, the layer tears, leaving stretch marks.
This type of scarring happens when the skin cannot bounce back after a period of intense growth, be that due to pregnancy, weight gain, or during
puberty. Extreme weight loss can also make stretch marks more visible, and some stretch marks arise as a result of acute trauma affecting the skin (such as a car accident when heavily pregnant).
Stretch marks are usually a slightly red or purple shade to begin with, fading with time to silvery white lines that may or may not disappear.
medicalnewstoday.com