Stretch marks are a type of scarring on the skin. These marks usually appear in shapes of bands, stripes or lines on particular parts of the human body such as the breasts, abdomen, arms and especially around the hips.
Collagen is the main structural protein in the skin. When there is a rapid stretching of the skin beyond its elasticity, the collagen layer tears causing white streaks.
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Article and image source: cuisineandhealth.com