What causes stretch marks?
Humans have three layers of skin: the epidermis/outer layer, dermis/middle, and the subcutaneous stratum/inner layer. Stretch marks appear within the dermis layer of the skin. Dermis helps skin retain its shape, so when it is continually stretched, the small connective fibres within the skin begin to break down, causing the skin to lose its elasticity. This then causes the appearance of stretch marks.