To be fair it is not completely understood but this is a simplified understanding of the current state of knowledge.

it is thought that you get stretch marks because the skin stretches with some weakness of the lower skin layers (dermis) often due to other factors( e.g. hormones).

This then causes the epidermis (top layer) to thin, allowing you to see the blood vessels underneath.

If you catch them when they are red or just as they begin I am told by may patients that Bio Oil is helpful. In conjunction with this there are some very encouraging results with the FRAXEL laser for RED stretch marks.

There is more info below