Moles, or nevi, consist of an abnormal collection of pigment cells on the outer layer of the skin, and can be round, oval, flat or raised. They can form alone or in groups.

Some people are born with moles, and others develop them over time – usually during childhood and the early adult years. Just about everyone has a few. In fact, the average person has between 10 and 40 moles.

About one to three percent of babies are be born with type of mole known as a congenital nevi. Other moles develop after birth. Some of these moles develop spontaneously; others are caused by exposure to sunlight.

Moles are extremely common, and in most cases aren’t a concern. The typical mole appears on the skin and can remain there for decades. Some moles will not change at all while others will fade and disappear over time.

While most moles are benign, there is a chance that they could develop into malignant melanoma, or skin cancer. This is most common with atypical moles (dysplastic nevi), which are most often bigger than a pencil eraser with irregular shape and pigmentation. Atypical moles are often hereditary.

People with large numbers of moles, especially atypical or those present at birth, are at higher risk for melanoma. Other risk factors include a family history of melanoma, changes in the mole’s color, height, size or shape and symptoms that include itching, pain, spontaneous bleeding and oozing at the mole site.

A checklist known as the ABCDE’s is commonly used to assess moles and their risk for melanoma. A stands for Asymmetry, meaning a mole is irregularly shaped, with one half not matching the other. B is Border, and comes into play when the edges of the mole are not smooth, but have a jagged or irregular appearance. The Color of the mole is also important. Color becomes a concern when one mole has different shades of tan, brown, black, blue, white or red within it. D equals Diameter. Moles that have a diameter larger than the eraser of a pencil are at higher risk. Finally, E is Elevation, meaning the mole is raised above the skin.

Any changes in the ABCDE’s of a mole should be assessed by a physician.

When it comes to your health and skin cancer, it’s best to be proactive. This includes limiting your exposure to sunlight and using a sunscreen when out in the sun.

You should also be familiar with your skin and the location of any moles that you have. Examine yourself periodically with a mirror (once a month if you have a family history of melanoma) to note changes or any new moles that have formed. The most common location for melanoma in men is the back; in women it is on the lower leg. However, it is important to examine every part of your body, even those hidden areas such as the groin and backs of your knees. Start at your head and work your way down to your feet (and even between your toes). Pay particular attention to areas of your body that are exposed to the most sunlight.

Hormones can cause changes in moles. Pay close attention to moles during puberty, pregnancy, menopause and other times when your hormones may be at elevated levels.

Skin moles are very common, and in most cases not the cause for concern. However, early detention and treatment of skin cancer can help prevent problems and lead to a positive outcome. It’s important to be familiar with your body, note any changes to moles and bring them to the attention of your health care professional.