Birthmarks form when skin cells, blood vessels, or pigment-producing cells develop irregularly during fetal growth or shortly after birth. They are not caused by anything a parent did, ate, or experienced during pregnancy. The two broad categories, vascular and pigmented, have different cellular origins but share a common thread: small, random changes in how cells grow and organize before or just after a baby is born.
Two Types With Different Origins
Vascular birthmarks are made of blood vessels that didn’t form correctly. They range from faint pink patches to raised, bright red marks to deep purple stains, depending on which blood vessels are involved and how close they sit to the skin’s surface.
Pigmented birthmarks are clusters of pigment cells that cause visible color changes in the skin. These cells, called melanocytes, are responsible for giving skin its color. When they group together in unusually high concentrations during development, the result is a flat or slightly raised mark that can be tan, brown, blue-gray, or black. In a large survey of 1,000 newborns, the most common pigmented birthmark was the Mongolian spot (appearing in about 67% of babies), followed by café-au-lait spots (30%) and congenital moles (about 2%).
What Goes Wrong With Blood Vessels
Vascular birthmarks arise from errors in the process that builds blood vessels during early development. These errors are almost always caused by somatic mutations, meaning they happen spontaneously in a small number of cells rather than being inherited from a parent. The mutation occurs early enough in fetal development that the affected cells multiply and form a visible patch, but late enough that only part of the body is involved. Scientists call this mosaicism: a person carries the mutation in some cells but not others.
Port-wine stains, the flat purple-red marks that don’t fade on their own, are the best-studied example. They’re caused by a specific mutation in a gene called GNAQ that keeps certain signaling proteins stuck in the “on” position, causing capillaries near the skin surface to stay permanently dilated. The proportion of cells carrying this mutation within a birthmark ranges from about 6% to 85%, and higher percentages tend to correlate with more severe marks. Over time, port-wine stains typically darken, thicken, and may develop a pebbly texture under the skin.
Salmon patches (sometimes called “stork bites” or “angel kisses”) are by far the most common vascular birthmark, showing up in roughly 36% to 44% of newborns. These are faint pink or red patches, most often on the forehead, eyelids, nose, or back of the neck. Unlike port-wine stains, they tend to fade on their own. Marks on the face are rarely detectable after age 5 or 6, though patches on the back of the neck can persist longer.
How Strawberry Marks Grow and Shrink
Infantile hemangiomas, the raised, bright red marks sometimes called strawberry birthmarks, follow a distinctive life cycle that sets them apart from other birthmarks. They’re usually not visible at birth or appear as just a faint discoloration with a pale halo. Over the first one to two weeks, they become more obvious.
From there, the mark enters a rapid growth phase, with the fastest expansion happening between about 5.5 and 7.5 weeks of age. Growth typically plateaus around 9 months, when the hemangioma reaches its maximum size. Then it slowly shrinks. In roughly 90% of cases, the mark has fully regressed by age 4. Some leave behind a small area of loose or slightly discolored skin, but many disappear entirely. Hemangiomas are relatively uncommon compared to salmon patches, occurring in about 1% of newborns.
Why Pigment Cells Cluster Together
Pigmented birthmarks form when melanocytes either gather in unusually dense groups or settle in deeper layers of skin than normal. Mongolian spots, the blue-gray patches common on the lower back and buttocks, happen when melanocytes end up in the dermis (the deeper skin layer) instead of the epidermis (the outermost layer) during fetal development. Because the pigment sits deeper, it scatters light differently and appears bluish rather than brown.
Congenital moles form when melanocytes cluster together closer to the surface. These cells often carry a specific mutation that drives their initial growth but also triggers a natural braking mechanism. The cells essentially enter a dormant state after forming the mole, which is why most congenital moles remain stable in size relative to the child’s growth. Café-au-lait spots, the flat light-brown patches, result from melanocytes in a given area producing more pigment than surrounding cells without necessarily forming a raised cluster.
The Role of Genetics
Most birthmarks are not inherited. The genetic changes responsible are somatic mutations that arise randomly during cell division in the womb. A parent doesn’t pass on a “birthmark gene,” and having a birthmark doesn’t mean your children will have one in the same spot or at all.
There are rare exceptions. Some families carry inherited mutations that make certain vascular birthmarks more likely. Mutations in a gene called RASA1 can run in families and cause clusters of small capillary malformations, sometimes alongside more complex blood vessel abnormalities. Another gene, PIK3CA, is linked to a group of overgrowth conditions where vascular marks appear alongside tissue overgrowth in a limb or other body region. But these inherited patterns account for a small fraction of all birthmarks. The vast majority are one-off events with no family connection.
When a Birthmark Signals Something Deeper
Most birthmarks are purely cosmetic and carry no health risk. But certain types, or certain patterns, can be associated with broader medical conditions. A port-wine stain on the upper face near the forehead and eye can be part of Sturge-Weber syndrome, which involves abnormal blood vessel development affecting the brain and eyes. Diagnosis is based on having at least two of three features: a facial port-wine birthmark, increased pressure in the eye, and abnormal blood vessel growth on the surface of the brain.
Café-au-lait spots are another example. One or two are common and harmless. But six or more spots larger than a certain size can be one of the early indicators of neurofibromatosis, a genetic condition affecting nerve tissue. This is why pediatricians often count and measure these marks during routine checkups.
A port-wine stain that becomes noticeably larger, raised, or thicker later in life can also signal developing complications and is worth having evaluated.
What Doesn’t Cause Birthmarks
For centuries, European folklore attributed birthmarks to “maternal impressions,” the idea that a mother’s cravings, emotions, or frightening experiences during pregnancy could physically mark the baby. Craving strawberries supposedly caused red marks. Witnessing something disturbing supposedly left a visible imprint. By the 18th century, physicians were already pushing back on this theory, noting there was no consistent evidence connecting a mother’s experiences to marks on her child. Modern genetics has confirmed that birthmarks arise from random cellular events during development, not from anything a pregnant person eats, feels, or does.