Yes, port wine stains do blanch when you press on them. This is actually one of the key clinical features used to confirm that a skin marking is a vascular malformation rather than a bruise or pigment-based birthmark. The color temporarily fades because pressure pushes blood out of the dilated blood vessels sitting just beneath the skin surface, and when you release the pressure, blood refills those vessels and the color returns.
How the Blanching Test Works
The formal name for this test is diascopy. A clinician presses a glass slide or clear object firmly against the skin over the suspected birthmark. If the red or purple color disappears under pressure, the test is considered positive, meaning the color comes from blood pooled in abnormal vessels rather than from pigment deposits, bleeding into the tissue, or other non-vascular causes. Port wine stains consistently show positive blanching on diascopy.
This works because port wine stains are made up of permanently widened, immature blood vessels sitting in the upper layers of the skin. These vessels lack the normal structure of healthy veins or arteries. When external pressure compresses them, the blood inside is temporarily displaced, and the overlying skin returns to its natural tone. Remove the pressure, and the vessels refill immediately.
Blanching May Change With Age
Port wine stains are present from birth and never disappear on their own. In childhood, they typically look like flat, light pink patches. Over time, though, they darken progressively from pink to red to deep purple as the blood vessels continue to widen, a process called vascular ectasia. By the fifth decade of life, roughly 65% of untreated port wine stains develop raised, thickened skin or nodules, based on a review of over 400 patients.
A young child’s flat, pink port wine stain blanches easily and completely with gentle pressure. As the lesion darkens and thickens with age, the deeper and more engorged vessels become harder to compress fully. The stain still blanches to some degree, but in older adults with nodular, deep purple lesions, blanching may be partial or harder to observe. The mean age at which noticeable thickening develops is around 37 years.
What Blanching Tells You (and Doesn’t)
Blanching confirms a lesion is vascular, but it doesn’t distinguish a port wine stain from every other vascular mark. Hemangiomas (raised, bright red growths common in infants) also blanch under pressure. So do salmon patches, the extremely common pink marks on newborns’ eyelids, foreheads, or the backs of their necks. The key differences lie in other features: salmon patches fade on their own within the first year or two, hemangiomas grow rapidly and then shrink over several years, while port wine stains are flat at birth, persist permanently, and progressively darken.
A mark that does not blanch is more likely a bruise, a pigmented birthmark, or bleeding beneath the skin (petechiae or purpura). This distinction matters because non-blanching rashes, especially in children with fever, can signal serious conditions that need immediate attention.
Port Wine Stains and Sturge-Weber Syndrome
Port wine stains affect about 0.3% of newborns. Most are purely cosmetic, but when a port wine stain covers the forehead or upper eyelid area (the region supplied by the first branch of the trigeminal nerve), it raises the possibility of Sturge-Weber syndrome. This condition involves abnormal blood vessels not just in the skin but also in the brain, the eyes, or both. A diagnosis requires a port wine stain plus at least two of three features: the birthmark itself, abnormal blood vessels in the eyes, and abnormal blood vessels in the brain. Blanching behavior alone cannot rule Sturge-Weber in or out; imaging and eye exams are needed.
How Laser Treatment Creates Blanching
Pulsed dye laser therapy, the standard treatment for port wine stains, works by targeting the blood inside dilated vessels. The laser energy heats and destroys the superficial abnormal vessels (those within about 300 micrometers of the skin surface), which produces visible blanching of the treated area. Over multiple sessions, this can lighten the birthmark significantly.
Results vary widely. Across decades of studies, only about 21% of all treated patients achieve 75% to 100% clearance. The best outcomes happen when treatment starts very early in life. Studies focusing exclusively on infants treated before six months of age showed notably better responses, with around 30.5% of previously untreated patients reaching that 75-100% clearance range. Deeper vessels that sit beyond the laser’s reach are a major reason complete clearance remains difficult, and port wine stains can partially regrow after treatment as remaining abnormal vessels continue to dilate over time.
Starting treatment in childhood, when the stain is still flat and pink with shallow vessels, gives the laser the best chance of producing lasting blanching. Waiting until adulthood, when the vessels have deepened and the tissue has thickened, makes the same degree of clearance harder to achieve.