According to the Mayo Clinic, scientists don’t understand why moles form or whether they have a purpose. However, it’s not uncommon for adults to have from 10 to 40 moles on their bodies, says the Cleveland Clinic. Most moles develop in childhood, but some develop later on. Although it’s normal for moles to change or even disappear, moles that look different or crop up suddenly in adulthood should be checked out.

What Causes Moles to Form

Melanin, a naturally occurring pigment providing skin color, produces cells called melanocytes, according to the Mayo Clinic. Sometimes, these cells cluster together for unknown reasons and cause moles. However, some scientists believe moles are caused from skin damaged by the sun, according to American Osteopathic College of Dermatology. Moles tend to get darker with more sun exposure, says the Cleveland Clinic, but sometimes, they get darker during puberty or pregnancy. Moles are common, though, and are usually harmless.

Changes in Moles

Although it’s normal for moles to change slightly or disappear in adulthood, if they change shape, or brand new moles quickly develop, it’s important to seek advice from your health-care provider who can determine your next step.

Use a mirror or ask a loved one to help you to inspect moles in places you can’t easily see, such as the back of your thigh. Double-check moles on skin regularly exposed to sun.

Moles and Cancer

Sometimes, moles turn to cancer. According to the Mayo Clinic, “several types of moles have a higher than average risk of becoming cancerous.” Congenital nevi--moles people are born with--could increase the risk of a fatal type of skin cancer called malignant melanoma. Finally, the more moles people have, the greater at risk they are for melanoma. Men are most likely to develop melanoma on their backs, whereas women are most likely to develop melanoma on their lower legs, says the Cleveland Clinic.

If your dermatologist decides you have a dangerous-looking mole, he or she will start by taking a biopsy of the mole, explains the Cleveland Clinic. It’s a safe and easy procedure, and if it’s determined that the mole is cancerous, it’s then carefully removed by a simple surgical procedure. If caught early and removed, cancer is not likely to spread.