Stretch marks will appear because of a rupture of collagen in the middle layer of your skin (the dermis) that is visible on the skin's surface. This rupture is caused when the fibers that make up the dermis are stretched as your bump grows and eventually they will stretch too far and break. This effect is increased during pregnancy, due to the pregnancy hormonal changes that soften the collagen ligaments of the pelvis. This allows the tissue to stretch easily during childbirth which has an effect on the skin's collagen as well.
Stretch marks can range a great deal in size varying from a few millimeters to over 8cm in length. While you're most likely to find them on your bump, they can also appear on your breasts, bottom or thighs. (check the link from here)