During the second and third trimesters your uterus will grow rapidly, stretching the skin over the stomach and tearing the connective tissues in the dermis. The torn tissue heals with irregular scars known as stretch marks (striae gravidarum). Growth of fat deposits in the breasts, hips, thighs, and bottom can cause stretch marks in these areas as well.
Stretch marks are usually caused by pregnancy, but they can also happen during puberty, or from certain hormonal imbalances.
Stretch marks affect up to 80% of all women. They occur more often in first pregnancies, the fair-skinned and the overweight. There is definitely a genetic component, but there are things you can do to minimize your risk.
Keep your weight gain within the guidelines recommended by your obGYN, to minimize the amount of stretching your skin has to do.