Simply put, stretch marks are caused by the over-stretching of the skin. This explains why they commonly affect people going through periods of rapid changes in size: bodybuilders, dieters, teenagers and pregnant women.
The skin is unable to keep up with the demands being placed on it, and the dermal layer eventually stretches beyond its elastic capacity. The result is a red or purple mark that can vary in size and colour.
While the skin will attempt to repair the damage, the resulting scar tissue differs in appearance and feel to the normal and healthy skin. This makes them rather noticeable, and although stretch marks can fade over time, they will never disappear completely.
It is thought that certain hormones also contribute to the appearance of stretch marks. Glutocorticoid hormones prevent the fibroblasts from producing collagen and elastin fibres. These provide the supportive structure that keeps skin elastic and taut, and low levels leave the skin unable to resist over-stretching and tearing.
Some creams and medications can also interfere in the production of this supportive structure, causing the skin to thin and become more prone to damage. While it is not always possible to prevent the appearance of stretch marks, stretch mark removal can significantly improve the way they look once they are there.