Stretch marks are the bane of every woman who has delivered a child. They are long, wide and shiny or silvery marks that happen on the stomach, breasts, buttocks and thighs. They happen because of the stretching that occurs during pregnancy as the baby continues to grow. Under the skin the collagen separates and the results are stretch marks. Because they happen as the skin stretches they often occur in the later stages of pregnancy as the baby is growing at a fast rate.