So you have developed mysterious little lines on your upper thighs. Have your children been drawing on you in your sleep? Are little green aliens slipping through your window in the dead of night to compose star maps on your legs?

Probably not. Instead, you have probably developed striae, better known as stretch marks. Before you reach for the stretch mark cream, you should understand how stretch marks come to be.

Despite the name, stretch marks are a scarring of the dermis, your middle layer of skin, by specific hormones called glucocorticoid hormones. These hormones inhibit the development of elastin and collagen fibers, which keep skin taut. Without this fiber support, your dermis will tear with any kind of stretching or expanding, even normal every day stretching could do the damage if you have a high concentration of these hormones.

So normal hormonal balance should help you keep stretch marks at bay. But sometimes, such as pregnancy and adolescence, it is very difficult to always keep those hormones in check.

So it may be easier to just blame those little lines on little green aliens.