Stretch marks are red, pink, or brown streaks on the skin. They are most common the belly, but also often they appear on the thighs, buttocks, abdomen, upper arms, and breasts. Most women get some stretch marks during 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 face. These spots are called melasma or chloasma and are more common in darker-skinned women.
These scars are caused by the stretching of the skin and the breakdown of protein in the skin by the high levels of pregnancy hormones, and usually appear in the second half of pregnancy.
Unfortunately, there's not much you can do to prevent stretch marks. Creams that you can rub into your skin usually have no effect. However, putting on weight gradually probably does help.
Be patient — stretch marks and other skin changes usually fade after delivery.