Coming in a variety of sizes and appearances, they may be irregular in shape and flat, or raised and lumpy in appearance and feel. Such naevi can also manifest themselves as beauty marks, which most commonly appear on the face, neck or arms. Café au lait spot
Café au lait spot macules may occur anywhere on the body. They are most commonly oval in shape and light brown, or milk coffee, in color. These birthmarks may be present at birth, or appear in early childhood, and do not fade much with age. One or two on an individual is common; however, four or more may be an indicator of neurofibromatosis. In the event of weight gain, the birthmark can stretch with the skin and become larger. Mongolian spot
A Mongolian blue spot (dermal melanocytosis) is a benign flat congenital birthmark with wavy borders and irregular shape, most common among East Asians and Turkic people (excluding Turks of Asia Minor), and named after Mongolians. It is also extremely prevalent among East Africans and Native Americans. It normally disappears three to five years after birth and almost always by puberty. The most common color is blue, although they can be blue-gray, blue-black or even deep brown. The Mongolian spot is a congenital developmental condition exclusively involving the skin. The blue colour is caused by melanocytes, melanin-containing cells, that are deep under the skin. Usually, as multiple spots or one large patch, it covers one or more of the lumbosacral area (lower back), the buttocks, sides, and shoulders.
Irregular in appearance, they are usually quite large, and caused by a deficiency or absence in the nerve supply to blood vessels. This causes vasodilation, the dilation of blood vessels, causing blood to pool or collect in the affected area. Over time, port-wine stains may become thick or develop small ridges or bumps, and do not fade with age. Such birthmarks may have emotional or social repercussions. Port-wine stains occur in 0.3% of the population, equally among males and females. They frequently express unilaterally, i.e., on only one side, not crossing the midline of the body. Often on the face, marks on the upper eyelid or forehead may be indicative of a condition called Sturge–Weber syndrome. Additionally, port-wine stains in these locations may be associated with glaucoma and seizures. Treatment
Most birthmarks are harmless and do not require treatment. Pigmented marks can resolve on their own over time in some cases. Vascular birthmarks may require reduction or removal for cosmetic reasons. Treatments include administering oral or injected steroids, dermatological lasers to reduce size and/or color, or dermatologic surgery. Historical Explanations
Many explanations were given to explain the origin of birthmarks. Occasionally, it was said that children could be marked or  imprinted upon by scares or frights given to the mother during the pregnancy.