Striae are caused by tearing of the dermis during periods of rapid growth of the body, such as during puberty or pregnancy, in which they usually form during the last trimester.

The most common places are the abdomen (especially near the navel), breasts, upper arms, underarms, back, thighs (both inner and outer), hips, and buttocks.

[5] Stretch marks occur in the dermis, the resilient middle tissue layer that helps the skin retain its shape.

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.

[citation needed] Examples of cases where stretch marks are common 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.

Common medications that can contribute to stretch marks include "corticosteroid creams, lotions and pills and chronic use of oral or systemic steroids".

[13] A number of additional factors appear to promote the appearance of stretch marks: 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.

[14] These skin marks are symptoms of pregnancy caused by the tearing of the dermis, resulting in atrophy and loss of rete ridges.

[citation needed] Mechanical distension and rapidly developing areas of the body during pregnancy (such as the abdomen, breasts, and thighs) are most commonly associated with striae formation.

Some have suggested that relaxin and estrogen combined with higher levels of cortisol during pregnancy can cause an accumulation of mucopolysaccharides, which increases water absorption of connective tissue, making it prime for tearing under mechanical stress.

[7][14] There also seems to be an association between higher body mass indices and in women with bigger babies and the incidence and severity of striae.

[21][22] Topical tretinoin is categorized by the United States Food and Drug Administration (FDA) as a known teratogen (causing malformations in fetuses) in animals, without adequate human studies on safety in pregnancy.