What are stretch marks?...

The lower layers of skin are made up of proteins called collagen and elastin, which give the skin its strength and stretchiness. When the skin stretches too much or too quickly, vertically, horizontally or both, for example, during pregnancy, these layers stretch and break in places.

Because there are then gaps in the layers of skin, the deep blood vessels can be seen, making the stretch marks appear a reddish brown or purple colour when they're new. Over time, the blood vessels contract and the pinkish colour changes to a silvery white, which is actually the colour of the fat underneath the skin.

In this way stretch marks do fade to become considerably less noticeable, but they are permanent scars.