Stretch marks come down to speed. Skin is sometimes asked to expand or shrink faster than it can build new collagen. When that happens, the support structure underneath gives way and a striae forms. A handful of situations account for almost all of them.
Pregnancy is the classic trigger. The abdomen and breasts stretch quickly in the later months. Meanwhile, hormones soften the skin’s connective tissue at the same time. Puberty is next. Rapid growth spurts leave marks across the thighs, hips, back and chest in teenagers of every build. Weight change in either direction plays its part, and so does the fast muscle gain of heavy training. That is why bodybuilders see them across the shoulders and arms. Genetics set the baseline underneath all of this. If your mother or sister marks easily, you are more likely to as well, because skin elasticity runs in families.
Less commonly, a course of strong steroid creams or a rise in the body’s own cortisol can thin the skin. This can bring marks on. That is one reason a sudden crop deserves a proper look. The NHS overview of stretch marks is a clear, plain-language summary of who tends to get them and why.
None of these causes are things to feel responsible for. They are simply the skin doing what skin does under load, and they respond well once the marks have settled.