They appear as bands of thinned skin, usually grouped in the abdomen, breasts, thighs and lumbosacral zone. Sudden changes in skin tension, due to weight gain or loss, facilitate their appearance. Skin can stretch to accommodate size changes, including those due to weight fluctuations, puberty, muscle gain, and pregnancy. If the skin stretches too quickly, the collagen and elastin connective tissues in the skin may break, resulting in stretch marks.