Mongolian blue spots, also known as slate gray nevi, are a type of pigmented birthmark. They’re formally called congenital dermal melanocytosis.

These marks are flat and blue-gray. They typically appear on the buttocks or lower back, but may also be found on the arms or legs. They’re generally present at birth or develop soon after.

These birthmarks are noncancerous and present no health danger. However, your child’s pediatrician should examine the marks to confirm the diagnosis. There’s no recommended treatment for Mongolian blue spots. They usually fade before adolescence.

Birthmarks are markings on the skin that show up around the time a baby is born. There’s no way to prevent them.

According to the Cleveland Clinic, birthmarks appear within two months after birth. If a mark shows up later in adulthood, it’s not considered a birthmark. Mongolian blue spots appear around the time of birth.

There are two main types of birthmarks: red (vascular) and pigmented birthmarks. Red birthmarks occur as a result of too many blood vessels. They can have many complications, such as bleeding and pain.

Pigmented birthmarks have no known causes, and many don’t cause any adverse health effects. Mongolian blue spots fall into this category of birthmarks. Other types of pigmented birthmarks may be associated with health risks, but Mongolian blue spots aren’t.

Mongolian blue spots appear on the skin at or shortly after birth. The spots appear when melanocytes (cells that produce pigment, or melanin) remain in the deeper skin layer during embryonic development. What causes this to happen isn’t known. Mongolian blue spots aren’t related to an underlying health condition.

Sometimes the spots are mistaken for symptoms of a common spinal condition called spina bifida occulta. However, according to the Spina Bifida Association, related spots are red — not the grayish color of Mongolian blue spots.

The amount of melanin (the substance responsible for skin color) you have generally determines the color of pigmented birthmarks. People with darker skin are more likely to have pigmented birthmarks.

The precise causes of Mongolian blue spots are unknown, as are risk factors that can increase your odds of getting them. However, melanin ultimately plays a role in any form of skin discoloration.

Mongolian blue spots seem to be more common in people with dark skin, including people of African, East Indian, or Asian descent.

Because of their color, Mongolian blue spots can be mistaken for bruises. They’re:

  • flat against the skin, with a normal skin texture
  • blue or blue-gray in color
  • usually 2 to 8 centimeters wide
  • an irregular shape, with poorly distinguished edges
  • usually present at birth or soon after
  • usually located on the buttocks or lower back, and less commonly on the arms or trunk

However, unlike bruises, Mongolian blue spots don’t disappear within a matter of days.

It’s important to understand the common characteristics of these spots. Marks likely aren’t related to Mongolian blue spots if they:

  • are raised
  • aren’t bluish
  • appear later in life

Mongolian blue spots are harmless. They’re not cancerous or indicative of a disease or disorder. There’s no need for medical intervention. In many cases, the spots fade over time and are gone by the time a child becomes a teenager.

If your child appears to have Mongolian blue spots, make sure the pediatrician examines them at your baby’s first checkup. A doctor can diagnose Mongolian blue spots based on their appearance.

The only possible complication of these spots is psychological. This is particularly the case for blue spots that are visible to others and last longer than childhood.

Most Mongolian blue spots fade over time. Like other types of noncancerous birthmarks, they don’t cause any long-term health problems.

Spots that start to change shape or color may be something else. Never self-diagnose any skin conditions. Always check with your doctor or a dermatologist.