Stretch marks are red, pink, or brown streaks on the skin. Most often they appear on the thighs, buttocks, abdomen, and breasts. These scars are caused by the stretching of the skin, and usually appear in the second half of pregnancy. 
Some women notice other skin changes during pregnancy. For many women, the nipples become darker and browner during pregnancy. Many pregnant women also develop a dark line (called the linea nigra) on the skin that runs from the belly button down to the pubic hairline. Patches of darker skin usually over the cheeks, forehead, nose, or upper lip also are common. Patches often match on both sides of the face. These spots are called melasma or chloasma and are more common in darker-skinned women. 

What might help: 
•    Be patient — stretch marks and other changes usually fade after delivery.