Stretch marks, typically on the abdomen and breasts, are actually tiny bands of scar tissue resulting from skin stretched too far or too fast. Like all scars, they may fade with time but don't completely disappear.
Darkened areas of skin are also common. On the cheeks and upper lip, this condition is called chloasma, or the "mask of pregnancy"; on the abdomen, it's a vertical line known as the linea negra. Both fade dramatically after you've given birth, but in rare cases, there may still be a faint trace of color left behind.
Moles may be darker or larger than they were before the baby -- usually harmlessly so, but any changed mole should be checked by a dermatologist.