Birthmarks form because of small, random irregularities during fetal development, either in blood vessels or in the cells that produce skin pigment. They are not caused by anything a parent did or didn’t do during pregnancy. Most newborns have at least one: in large surveys, roughly 80% to 90% of babies are born with some type of visible skin marking, with the most common being bluish-gray patches on the lower back and pink “salmon patches” on the eyelids, forehead, or neck.
How Pigmented Birthmarks Form
The cells responsible for skin color, called melanocytes, originate near the developing spinal cord in a structure called the neural crest. Early in fetal development, these cells break away and migrate outward through the body to take their positions in the skin. They follow a specific path, traveling between layers of developing tissue before settling into the outer skin and hair follicles, where they begin producing melanin.
Pigmented birthmarks happen when this migration doesn’t go perfectly. Some melanocytes may stop short of their destination, clustering in the deeper layers of skin instead of spreading out evenly. Others may multiply more than usual in one spot. The result is a visible patch of darker or differently colored skin. Café-au-lait spots (flat, light-brown patches), congenital moles, and bluish-gray patches on the lower back all form through variations of this process.
Bluish-gray birthmarks, sometimes called congenital dermal melanocytosis, appear blue rather than brown because of how light interacts with pigment trapped deep in the skin. Shorter wavelengths of light scatter off these deep melanocytes and reflect back as blue-gray, similar to why veins look blue through the skin. These marks show striking differences across ethnic groups: they appear in 90% to 100% of Asian and African newborns, about 50% of Hispanic infants, and roughly 10% of White infants. One explanation is that melanocyte production in the deeper skin layers lasts longer during fetal development in some populations, so the pigment is still present at birth rather than having already faded.
How Vascular Birthmarks Form
Vascular birthmarks are caused by abnormal blood vessel development rather than pigment cells. They come in two main varieties: malformations (where blood vessels form incorrectly) and hemangiomas (where blood vessel cells multiply too aggressively).
Port-wine stains, the flat reddish-purple patches that are present from birth, result from an increased number and size of tiny blood vessels just beneath the skin’s surface. Research has traced most of these to a specific genetic accident: a somatic mutation in a gene called GNAQ. This mutation isn’t inherited from either parent. Instead, it occurs spontaneously in a single cell during early fetal development, and that cell’s descendants carry the error forward as they build blood vessels in one area of the body. The mutation essentially locks a signaling protein into its “on” position, causing blood vessels to dilate and proliferate abnormally. About 80% of patients with port-wine stains affecting the face and brain carry this particular GNAQ change.
Hemangiomas, sometimes called “strawberry marks,” work differently. The blood vessel lining cells in a hemangioma are clonal, meaning they all descend from a single mutated ancestor cell. These cells migrate and multiply about three times faster than normal blood vessel cells, and they resist the body’s usual signals to stop growing. Scientists believe a somatic mutation in a gene related to new blood vessel growth triggers this runaway expansion. Unlike port-wine stains, hemangiomas are not always visible at birth. They typically appear in the first few weeks of life, grow rapidly during the first year, and then slowly shrink on their own.
The Role of Random Genetic Errors
The key concept behind most birthmarks is “somatic mosaicism.” Unlike inherited genetic conditions that affect every cell in the body, the mutations behind birthmarks happen in a single cell at some point after conception. That cell divides and passes the mutation to its descendants, but the rest of the body’s cells remain unaffected. The earlier in development the mutation occurs, the larger the birthmark tends to be, because that one altered cell has more time to produce a bigger population of daughter cells.
This is why birthmarks appear in random locations and sizes. It’s essentially a matter of chance: which cell mutated, when during development it happened, and where in the body that cell’s descendants ended up. There is no known way to prevent these spontaneous mutations, and they are not caused by anything during pregnancy.
What Didn’t Cause Your Birthmark
Folklore in many cultures links birthmarks to a mother’s food cravings, emotional experiences, or physical actions during pregnancy. The American Academy of Dermatology states directly that birthmarks don’t form because a pregnant woman ignores a food craving, touches her belly while worried, or eats certain foods. These are myths with no scientific basis. You cannot prevent birthmarks by satisfying cravings or avoiding stress.
When Birthmarks Deserve Attention
The vast majority of birthmarks are harmless. Salmon patches, the most common vascular birthmark (found in about 28% of newborns), typically fade on their own within the first year or two. Bluish-gray patches on the lower back almost always disappear by school age.
Hemangiomas follow a predictable timeline. They grow rapidly in the first year, with peak growth spurts around one to two months and again around four to five months. The shrinking phase generally begins around 18 months. About 50% resolve completely by age five, and 70% by age seven. Some continue to improve gradually through age 10 to 12.
A few types of birthmarks can signal something worth monitoring. Café-au-lait spots are common and usually harmless on their own, but six or more spots measuring at least 5 millimeters across in a child (or 15 millimeters in a teenager or adult) is one of the diagnostic criteria for neurofibromatosis type 1, a genetic condition affecting nerve tissue. A single spot is rarely a concern.
Large congenital moles, the dark brown or black birthmarks present from birth, carry a small but real increased risk of developing melanoma. Estimates for large congenital moles range from about 1.25% to 10% lifetime risk, which is dramatically higher than the general population’s risk. These moles also carry a slight chance of melanoma developing in the central nervous system rather than the skin. Dermatologists typically recommend long-term monitoring for anyone born with a large congenital mole.
Treatment Options for Persistent Birthmarks
Port-wine stains do not fade on their own and can darken or thicken over time. The standard treatment uses a pulsed dye laser, which targets the hemoglobin inside the abnormal blood vessels. In a large study of 848 patients, about 70% showed meaningful improvement, though complete clearance was much less common at around 6%. Most patients required an average of six treatments, with better results coming after five or more sessions. The range in that study was 3 to 20 treatments, reflecting how much individual results can vary based on location, size, and skin type.
Hemangiomas that are small and located away from the eyes, nose, mouth, or airway are usually left alone, since most shrink on their own. When treatment is needed, oral medication can slow growth during the proliferative phase. For birthmarks that are purely cosmetic concerns, laser treatments and minor procedures are options once the birthmark has stabilized.