MONGOLIAN SPOT
Notice the characteristic color that is actually easy to tell its not a bruise. Notice the border is not very sharp or distinct.
This is a normal birthmark. It is simply increased pigment in the skin. It is much more common in blacks, Asians and those with Mediterranean ancestry. Most are found on the back above or on the buttocks. They are commonly mistaken for bruises, but a mongolian spot has less distinct borders and a very characteristic color that should not be mistaken for a bruise.
This is not contagious.
Most mongolian spots resolve within the first 3 to 8 years of a child's life.
No treatment is necessary.
BRUISES: Bruises have more distinct borders and have a color that changes from red to blue black to yellow / green to brown and generally resolve in 7 to 21 days.
MOLES: Moles are brown, not blue black, have sharp borders and do not fade with time. A large version covering the buttocks area is called a bathing trunk nevus and this looks like a very large hairy mole.
I would ask your pediatrician at the next office visit about this if your are concerned, but generally the pediatrician would have told you about the spot at the newborn visit. I can't actually think of a reason to call a doctor for this.
