Most moles are completely harmless, but a small number develop into melanoma, a serious form of skin cancer. The key is knowing which visual changes signal trouble. Dermatologists use a simple five-feature checklist called the ABCDE rule, along with a few additional red flags, to sort normal moles from ones that need a closer look.
The ABCDE Rule
This is the standard framework doctors use, and it works just as well at home in front of a mirror.
- Asymmetry. If you drew a line down the middle of the mole, the two halves wouldn’t match. Normal moles are roughly symmetrical.
- Border. The edges are ragged, notched, or blurred rather than smooth and distinct. Pigment may look like it’s leaking into the surrounding skin.
- Color. Instead of one uniform shade, you see a mix of brown, tan, black, or even patches of white, gray, red, pink, or blue within the same mole.
- Diameter. The mole is wider than about 6 millimeters, roughly the size of a pencil eraser. Melanomas can be smaller than this, but most exceed that threshold.
- Evolving. The mole has changed in size, shape, color, or texture over the past few weeks or months. Any visible change matters, even if the mole doesn’t check the other boxes.
No single feature on its own confirms melanoma, but the more boxes a mole checks, the more urgently it should be examined by a dermatologist.
The “Ugly Duckling” Sign
Beyond the ABCDE criteria, dermatologists rely on a broader comparison: does one mole look noticeably different from all the others on your body? Most of your moles share a general “family resemblance” in color, size, and shape. A mole that stands out as the odd one, the ugly duckling, deserves attention even if it doesn’t obviously meet the ABCDE criteria. Researchers at Harvard’s Wyss Institute built an algorithm around this concept and found it agreed with dermatologists’ assessments about 88% of the time. In practice, this means stepping back and scanning your skin as a whole rather than just examining each mole in isolation.
Normal Moles vs. Atypical Moles
A normal (common) mole is typically smaller than 5 millimeters, round or oval, evenly colored in pink, tan, or brown, and has a smooth surface with a clean edge separating it from the surrounding skin. It may be flat or dome-shaped, but its overall appearance is tidy and uniform.
An atypical mole (sometimes called a dysplastic nevus) blurs those boundaries. It’s often wider than 5 millimeters, has an irregular or notched edge that fades into the skin around it, and displays a mixture of tan, brown, red, or pink shades. The surface can feel slightly scaly or pebbly. Atypical moles aren’t cancer, but having several of them increases your overall melanoma risk and makes regular monitoring more important.
Symptoms You Shouldn’t Ignore
Visual changes aren’t the only warning. A mole that starts itching, bleeding, or crusting without an obvious cause (like nicking it while shaving) can be a sign of melanoma. The same goes for a mole that begins to ooze or develops an open sore that won’t heal. It’s normal for a raised mole to get irritated by clothing or jewelry rubbing against it, but persistent or unexplained symptoms in a mole that used to cause no problems are worth having checked.
Spots to Check on Darker Skin
Melanoma in people of color most often appears in places that don’t get much sun. Acral lentiginous melanoma, the most common type of melanoma among people of color, shows up as a black or brown discoloration on the palms of the hands, the soles of the feet, or under fingernails and toenails. A dark streak running the length of a nail is a classic warning sign. These areas are easy to overlook during a casual skin check, which is one reason melanoma in darker skin tones is often caught at a later stage.
How to Do a Monthly Skin Check
The American Cancer Society recommends examining your skin once a month. You’ll need a full-length mirror, a hand mirror, and good lighting. A systematic approach makes it easier to catch changes early.
Start facing the wall mirror. Look at your face, ears, neck, chest, and abdomen. Women should lift their breasts to check the skin underneath. Next, examine your underarms, both sides of your arms, the tops and palms of your hands, between your fingers, and under your fingernails.
Sit down and check the fronts of your thighs, shins, tops of your feet, between your toes, and under your toenails. Then use the hand mirror to see the bottoms of your feet, calves, and backs of your thighs. Use the hand mirror angled against the wall mirror to inspect your buttocks, genital area, lower and upper back, and the back of your neck and ears. Finally, use a comb or hair dryer to part your hair section by section so you can see your scalp.
People at higher risk, including anyone who has had skin cancer before, has a large number of moles, has reduced immunity, or has a strong family history of skin cancer, should be especially consistent about monthly checks and may benefit from more frequent professional skin exams.
How Fast Melanoma Develops
Most melanomas grow slowly, changing over the course of several years in ways that can be spotted by eye if you’re paying attention. That’s the good news: regular self-checks give you a wide window to catch something early. In rare cases, though, melanoma can develop in just a few months, which is why the “evolving” criterion matters so much. Any noticeable change in a mole over a short period, whether in size, color, shape, or texture, is reason enough to have it evaluated promptly.
What Happens at the Dermatologist
If you bring a suspicious mole to a dermatologist, the first step is usually a close-up examination with a dermatoscope, a handheld magnifying device with a built-in light. Dermoscopy improves melanoma detection accuracy by roughly 49% compared to looking with the naked eye, though only when used by experienced clinicians.
If the mole still looks concerning, the next step is a biopsy. A shave biopsy removes a thin surface layer of skin with a razor blade and typically doesn’t require stitches. A punch biopsy uses a small circular tool to take a deeper sample and usually needs a stitch or two to close. Which type your doctor chooses depends on the mole’s depth and characteristics. The tissue sample goes to a lab, and results generally come back within one to two weeks.