This is particularly the case when there is new tissue growth (which can interfere with the underlying physical support of the dermis or epidermis, by displacing the supportive tissue).Examples of cases where stretch marks are common, also given by the Mayo Clinic, include weight gain (in the form of fat and/or muscle), pregnancy, and adolescent growth spurts, though it is also noted that some medications, as well as other medical conditions and diseases, may increase the likelihood of stretch marks appearing. In the case of medication, the Clinic points to "corticosteroid creams, lotions and pills and chronic use of oral or systemic steroids" as a common contributing factor. Medical conditions that can contribute to stretch marks include Ehlers-Danlos syndrome, Cushings syndrome, Marfan syndrome, and "adrenal gland diseases". Pregnancy
Pregnancy stretch marks, also known as striae gravidarum, are a specific form of scarring of the skin of the abdominal area due to rapid expansion of the uterus as well as sudden weight gain during pregnancy. About 90% of pregnant women are affected.A number of additional factors appear to promote the appearance of stretchmarks: one study of 324 women, done just after they had given birth, demonstrated that low maternal age, high body mass index, weight gain over 15 kg (33 pounds) and higher neonatal birth weight were independently correlated with the occurrence of striae. Teenagers were found to be at the highest risk of developing severe striae.These skin marks are symptoms of pregnancy caused by the tearing of the dermis, resulting in atrophy and loss of rete ridges.
No stretch marks will form as long as there is support within the dermis; stretching plays a role in where the marks occur and in what direction they run, however, there are a number of contributing factors (see: "Causes", below) to their formation.They can (but do not always) cause a burning and itching sensation, as well as emotional distress. They pose no health risk in and of themselves, and do not compromise the bodys ability to function normally and repair itself, however, they are often considered a cosmetic nuisance. Young women are generally affected the most and often seek treatment for them from a dermatologist and following pregnancy. Causes
Stretch marks appear to be caused by stretching of the skin. This is especially true when there is an increase in cortisone –  an increase in cortisone levels can increase the probability or severity of stretch marks by reducing the skins pliability. More specifically, it affects the dermis by preventing the fibroblasts from forming collagen and elastin fibers necessary to keep rapidly growing skin taut; this can create a lack of supportive material as the skin is stretched, and lead to dermal and epidermal tearing, which in turn can produce scarring in the form of stretch marks.