When parents first notice a bluish or grayish patch on their newborn’s skin, particularly across the buttocks or lower back, it can be concerning. This distinctive discoloration is a common, harmless phenomenon, especially among infants of non-European ancestry. The birthmark is present at birth or appears shortly after, and its bruise-like appearance often prompts inquiry.
The Medical Terminology and Appearance
The official medical term for this type of birthmark is Dermal Melanocytosis, although it is also widely known as a Mongolian Spot. The discoloration presents as a flat patch on the skin, ranging from slate-gray or blue-gray to blue-green or even black. Unlike a bruise, the texture of the affected skin is entirely normal and smooth to the touch.
The patches vary significantly in size, often measuring a few centimeters but sometimes covering a large area of the lower back and buttocks. The most common location is the sacral-gluteal region—the base of the spine and the buttocks—explaining the frequent observation of “blue bottoms.” Dermal Melanocytosis refers to the presence of pigment-producing cells within the deeper layer of the skin. Parents and caregivers should recognize that these are birthmarks, not the result of injury or bruising.
Understanding the Pigmentation Process
The appearance of Dermal Melanocytosis relates directly to the movement of specialized cells called melanocytes during fetal development. Melanocytes are the pigment-producing cells responsible for creating melanin, the substance that gives skin and hair its color. During early pregnancy, these cells originate from the neural crest and begin a specific migration path.
Melanocytes are supposed to travel from the neural crest and settle in the epidermis, the outermost layer of the skin. In Dermal Melanocytosis, some melanocytes become “trapped” or arrested in the dermis, the deeper layer of the skin, before completing their journey. This collection of pigment cells stuck in the dermis forms the birthmark.
The characteristic blue or gray color is an optical effect, known as the Tyndall effect, not the actual color of the pigment itself. When light hits the skin, the pigment deep within the dermis scatters the shorter, blue wavelengths of light more effectively than red wavelengths. This selective scattering makes the deep-seated brown-black melanin appear blue-gray when viewed through the layers of the skin, similar to how the deep ocean appears blue.
Who Gets Them and When Do They Fade?
Dermal Melanocytosis is common across many populations, with the highest frequency found in infants of Asian and Black descent. Studies show that these birthmarks are present in 81% to 100% of Asian infants and 95.5% to 96% of Black infants. They are also highly prevalent in infants with Native American and Hispanic ancestry, affecting between 46% and 70% of those populations.
The prevalence is lower in infants of White descent, occurring in roughly 10% of that population. These birthmarks are benign, meaning they are harmless and not associated with medical complications. No treatment is required, as the vast majority of these spots resolve naturally over time.
The prognosis is excellent, with the birthmarks typically fading and disappearing completely during early childhood. Most spots are gone by the time a child reaches five or six years of age. While they can occasionally persist into adolescence or, rarely, adulthood, this is generally seen with larger patches.