Port wine birthmarks are caused by a random genetic mutation that occurs during early fetal development, affecting the cells that line blood vessels in the skin. Unlike many genetic conditions, they aren’t inherited from parents. They affect less than 1% of newborns and are present from birth, appearing as flat patches of purple or dark red skin, usually on one side of the face or neck.
The GNAQ Gene Mutation
The root cause is a somatic mutation, meaning it happens spontaneously in a small group of cells rather than being passed down through family lines. The mutation occurs in a gene called GNAQ (or, less commonly, in a related gene called GNA11). Researchers have identified several specific variants of this mutation, with the most common being a change at position 183 of the GNAQ protein.
This gene normally helps regulate how blood vessel cells grow and behave. When it mutates, the affected cells receive a constant “on” signal that disrupts the normal functioning of the vascular endothelium, the thin layer of cells lining the inside of blood vessels. The result is capillaries and small veins that dilate abnormally and never learn to constrict properly. Because the mutation only affects a patch of cells rather than the entire body, the birthmark appears in one localized area.
The mutation is not something either parent carries or passes on. It arises randomly during cell division in the womb, which means there’s no known way to prevent it and no increased risk in future pregnancies.
Why the Blood Vessels Stay Dilated
The gene mutation alone doesn’t fully explain why port wine stains persist and worsen. Research points to a second factor: the nerve supply around the affected blood vessels is significantly reduced. Normally, tiny nerves surrounding capillaries in the skin send signals that keep vessel walls toned and appropriately constricted. In port wine birthmarks, the density of this surrounding nerve tissue is markedly lower than in normal skin.
Studies using confocal microscopy have found an inverse relationship between nerve density and blood vessel diameter. The fewer nerves surrounding a vessel, the wider it becomes. Areas with the lowest nerve density also tend to have the highest concentration of blood vessels and, notably, respond least well to laser treatment. Animal research confirms that the sympathetic nervous system plays a direct role in shaping how vessel walls develop during fetal growth, so reduced innervation during this critical window likely contributes to the permanent dilation that defines these birthmarks.
How They Differ From Temporary Birthmarks
Many parents confuse port wine stains with salmon patches, which are far more common. Salmon patches (sometimes called “stork bites” on the neck or “angel’s kisses” on the forehead) appear in roughly one out of every three newborns. They look like flat pink or red areas with blurry borders, become more visible when a baby cries or gets warm, and typically fade within the first year of life.
Port wine stains look different from the start. They’re darker, often purple or deep red, with well-defined borders. They don’t fade with time. Instead, they grow proportionally with the child and persist into adulthood. Over the years, the skin in the affected area usually gets darker and thicker, eventually developing a pebbly or bumpy texture as the dilated vessels continue to expand.
How They Change Over Time
Port wine birthmarks are not static. In infancy, they’re flat and smooth. Without treatment, they typically darken from pink to deep purple over the course of years or decades. The skin gradually thickens as the blood vessels beneath it continue to dilate and multiply. Many adults with untreated port wine stains describe the texture as feeling like pebbles under the skin. In some cases, the surface becomes raised and develops small, rounded nodules called blebs that can bleed easily if bumped or scratched.
This progression happens because the underlying vascular problem is ongoing. The mutated cells keep signaling for vessel growth and dilation, and without adequate nerve input to counteract that process, the vessels expand slowly over a lifetime.
When a Port Wine Stain Signals Something More
Most port wine birthmarks are isolated skin findings with no effect on internal organs. But in a small number of cases, the same type of vascular malformation extends beyond the skin.
Sturge-Weber Syndrome
When a port wine stain covers the upper part of the face, particularly the forehead and eyelid area, it can be associated with Sturge-Weber syndrome. In this condition, abnormal blood vessels also develop along the surface of the brain and in the eye on the same side as the birthmark. This can lead to seizures, glaucoma, and developmental differences. Not every child with a facial port wine stain has Sturge-Weber syndrome, and many children with birthmarks in this area have no additional symptoms at all. Doctors typically evaluate with brain imaging and eye exams to check for vessel abnormalities when the birthmark’s location raises concern.
Klippel-Trenaunay Syndrome
When a port wine stain appears on a limb rather than the face, it can occasionally be part of Klippel-Trenaunay syndrome. This condition has three hallmark features: the port wine stain itself, overgrowth of bone and soft tissue in the affected limb, and malformations of deeper veins. The overgrowth usually becomes apparent in infancy and is most often limited to one leg, though it can affect an arm or, rarely, the torso. The affected limb may become noticeably larger than its counterpart, causing pain, heaviness, and difficulty walking if one leg ends up longer than the other.
Vein malformations in Klippel-Trenaunay syndrome often include painful varicose veins near the skin surface and abnormalities in deeper veins that increase the risk of blood clots. Other possible complications include lymphedema (fluid buildup causing swelling), recurrent skin infections, and internal bleeding from abnormal vessels.
Imaging and Treatment Planning
Newer imaging tools are changing how doctors evaluate port wine birthmarks before treatment. Optical coherence tomography (OCT) can map the blood vessels beneath a birthmark without any needles or incisions. It measures vessel density and diameter at tiny increments of depth, from the skin surface down to about half a millimeter. Research has found that the highest concentration of abnormal vessels sits relatively shallow, between 0.15 and 0.2 millimeters deep.
This kind of detailed mapping matters because it helps explain why some areas of a birthmark respond better to laser treatment than others. Regions with vessels closer to the surface are generally easier to treat, while areas with deeper or denser vessel networks, particularly those with low surrounding nerve density, tend to be more resistant. Personalized treatment plans based on OCT mapping are an active area of clinical interest, with the goal of tailoring laser settings to each patient’s specific vessel depth and distribution rather than using a one-size-fits-all approach.