Knowledge center. Birthmarks are blemishes on the skin that are noticeable at birth, or shortly afterwards. A significant proportion of newborns have a vascular birthmark-a red, pink or purple blemish that is caused by abnormal blood vessels under the skin.Some people are born with pigmented birthmarks; these are usually brown and are caused by the clustering of pigment cells.n some cases the skin may become stretched or deformed, especially if it is a large mark. Telangiectatic nevus (salmon patch, stork mark/bite) - caused by dilations (expansion) in the capillaries (tiny blood vessels). They are patches of slightly reddened skin.