Over time they may diminish, but will not disappear completely. Stretch marks formed during pregnancy, usually during the last trimester, and usually on the belly, but also commonly occurring on the breasts, thighs, hips, lower back and buttocks, are known as striae gravidarum. Stretch marks are caused by tearing of the dermis. This is often from the rapid stretching of the skin associated with rapid growth or rapid weight changes. Stretch marks may also be influenced by hormonal changes associated with puberty, pregnancy, bodybuilding, or hormone replacement therapy.