Mongolian spots are very common in any part of the body of dark-skinned
babies. They are flat, gray-blue in color (almost looking like a bruise), and
can be small or large. They are caused by some pigment that didn't make it to
the top layer when baby's skin was being formed. They are harmless and usually
fade away by school age.
In most cases, no treatment is needed for the birthmarks themselves. When
birthmarks do require treatment, however, that treatment varies based on the
kind of birthmark and its related conditions. Read more about
newborn's skin and rashes.