The mystery of birthmarks has left its own mark in some superstitious beliefs of different cultures. In Italian and Spanish, the word birthmark means “cravings,” supporting an old wives’ tale — spots form when pregnant women ignore their food cravings. However, in Mental Floss’ latest video, “ Why Do People Have Birthmarks?” host Craig Benzine explains people are born with birthmarks, either due to extra pigmentation, the clustering of blood vessels, or because they simply appear.

Birthmarks fall into two categories: Pigmented and vascular. Pigmented birthmarks occur when the skin's pigment cells are clustered together, such as with moles and “café-au-lait” spots, which are usually light brown spots. Vascular birthmarks appear on the skin — the head and neck area, mainly on the face — when a group of blood vessels underneath the skin gather together in the same spot. However, both types of birthmarks can appear anywhere, including inside the body.

Examples of vascular birthmarks include hemangiomas — superficial hemangioma, including strawberry hemangioma, which are bright red and stick out of the skin, and deep hemangiomas (cavernous hemangioma), which are beneath the outer layer of skin, and appear bluish-red. Port wine stains, reddish spots that occur from wider, not more blood vessels, are also a type of vascular birthmark.

This explains why birthmarks appear, but since there are so many different types, and they appear in so many different ways, researchers still aren't sure about their exact cause, purpose, or why some are hereditary and why some aren't. However, there are a few theories about why birthmarks exist.

Birthmarks may be linked to certain proteins that a newborn is exposed to in the placenta, but the mechanism is still poorly understood. Meanwhile, hemangioma has been associated with low birth weight. A 2008 study concluded that a lack of oxygen in the placenta during embryo gestation could be responsible for the formation of hemangiomas.

Although these mystery marks are typically harmless, they are removed if they’re associated with medical problems, such as affecting the ability to see, breathe, or hear. If a birthmark is getting larger on an internal organ, that's a concern, or if it's growing really fast, it might be indicative of a tumor. In other circumstances, doctors will use laser surgery to treat birthmarks so that they can't be seen as much.