Causes
A number of causes promote the appearance of stretchmarks: one study of 324 women just after giving birth demonstrated that low maternal age, high body mass index, weight gain over 15 kg (33 pounds) and higher neonatal birth weight were independently associated with the occurrence of striae. Teenagers are at highest risk of developing severe striae.
The glucocorticoid hormones responsible for the development of stretch marks affect the dermis by preventing the fibroblasts from forming collagen and elastin fibers, necessary to keep rapidly growing skin taut. This creates a lack of supportive material, as the skin is stretched and leads to dermal and epidermal tearing.
Skin subjected to more stretching force than it can handle will tear. Hormonal changes and genetics influence the skin's capacity to withstand stretching forces, as do diet and (possibly) exercise.
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