Stretch marks(or stria) are usually caused by rapid weight gain in a short period of time, pregnancy or muscle enlargement. Stretch marks are tiny tears in the dermis (the elastic, resilient middle layer) that form when the skin stretches. Stretch marks can form anywhere on the body where the skin is stretched but they most commonly form on the breasts(for women), hips, abdomine, inner thighs, buttox, behind the knees and on the arms. Stretch marks start out as a reddish/purpleish colour but with time they heal and fade to whiteish coloured scars. Stretch marks mostly occur in women during pregnancy or girls going through puberty and growth spurts. They can also occur in men who gain weight, strength train(common among body builders)and can happen during growth. They are perfectly natural and very common so don't be ashamed if you have them-- most women and men do. Unfortunately there is not anything that can completely or permantely remove or get rid of stretch marks. Certain creams can help lighten the appearance of them but they will always be there. Ones on the abdomine or arm can be removed with a tummy tuck/arm tuck and toning the parts of the body with stretch marks can help tighten the skin, making them less visible. Until scientists discover a definite way to get rid of them, they are there forever. So learn to accept them as part of your body. Many people stress over these marks but they have no control over how their body adjusts to changes like this, so why stress? They are just scars that will become nearly invisible with time.
My mother has stretch marks on her abdomine after giving birth to two children.