The most common cause of stretch marks is underlying tissue expansion, which can occur during periods of rapid growth, such as puberty or pregnancy. Stretch marks commonly develop during the third trimester of pregnancy as the abdomen expands to accommodate the growing fetus. They can also occur in individuals who experience rapid weight gain or loss, such as bodybuilders or those with obesity. Certain medical conditions, such as Cushing's syndrome, where the body produces excess cortisol, can also increase the risk of developing stretch marks.