Moles form when melanocytes, the cells that produce your skin’s pigment, cluster together instead of spreading evenly throughout the skin. Most adults have between 10 and 40 moles, and the majority are completely harmless. They’re not designed to protect you or serve a biological function. They’re essentially the result of a minor glitch in how pigment cells grow and communicate.
What Happens Inside Your Skin
Melanocytes normally sit in the bottom layer of your outer skin, spaced apart from one another. They have a built-in behavior called contact inhibition: when one melanocyte bumps into another, both stop moving in that direction. This keeps pigment cells distributed evenly, which is why your skin tone looks uniform across most of your body.
A mole appears when something overrides that spacing mechanism. A single melanocyte picks up a genetic mutation that tells it to keep dividing, and its daughter cells stick together in a tight cluster rather than spreading out. That cluster produces pigment in a concentrated spot, creating the brown or tan dot you see on the surface. The most common mutation behind this process occurs in a gene called BRAF. Research published in the Journal of the National Cancer Institute found that this specific mutation is fully clonal in moles, meaning every cell in a given mole traces back to a single original cell that acquired the change. This tells scientists that a BRAF mutation is typically the very first event that kicks off mole formation.
Here’s what makes moles interesting: the same BRAF mutation is also the most common driver mutation in melanoma. Yet the vast majority of moles never become cancerous. After the initial burst of growth, the clustered cells stop dividing. Recent research from eLife suggests this growth arrest isn’t caused by the cells sensing something is wrong. Instead, the cells appear to respond to the same feedback signals that control the size of normal tissues, essentially the surrounding skin telling the cluster “that’s big enough.” This built-in braking system is why most moles stay small and stable for years.
Sun Exposure Is the Biggest Driver
Ultraviolet radiation is the strongest environmental factor in mole development. UV light increases the density of melanocytes in your skin and raises the odds of triggering the mutations that lead to clustering. Most moles show up on parts of the body that get the most sun.
This effect is especially pronounced in childhood. A study from the Colorado School of Public Health found that very light-skinned children who tan develop more moles than children who don’t tan. The connection runs deeper than just sun exposure alone: lighter hair color, lighter eye color, a tendency to freckle, and a history of sunburns all correlate with higher mole counts. People with darker skin tones tend to develop fewer moles overall.
Moles You’re Born With vs. Moles That Appear Later
A small number of moles are congenital, meaning they’re present at birth or show up in the first few weeks of life. These form during fetal development when melanocyte precursor cells migrate through the embryo and cluster before the skin is fully formed. Congenital moles are fairly common and may have a heritable component, though the genetics aren’t fully mapped.
The rest are acquired moles, and they accumulate over time. New moles appear most rapidly during adolescence. A four-year study tracking students who started at ages 12 to 14 found that their mole counts increased by 47% in the first year alone, with smaller increases in subsequent years. The count typically peaks somewhere in the 20s to 30s, then gradually declines as you age.
That decline is worth noting because it means moles aren’t permanent. The same study found a measurable turnover even in young people: moles were both appearing and disappearing throughout the tracking period. Most of the moles that came and went were small and flat, though some raised moles also faded. By older adulthood, many people notice they have fewer moles than they did in their 30s.
How Hormones Affect Your Moles
Hormonal shifts during puberty and pregnancy can influence moles, though the effect is more subtle than people often assume. The medical literature has long stated that moles darken and enlarge during pregnancy, and there’s some basis for this. One study tracking pregnant women with atypical moles found that the rate of visible change was about four times higher during pregnancy compared to non-pregnant periods. Researchers have attributed this to hormonal effects on melanocyte activity, particularly during the third trimester.
That said, when actual measurements are taken, dramatic changes are uncommon. In one study of 129 moles monitored through pregnancy, only about 6% changed in diameter at all, and those changes were just 1 millimeter in either direction. So while pregnancy can make some moles slightly darker or larger, most moles stay the same. The low incidence of melanoma before puberty and the known connection between skin pigmentation and hormones like estrogen suggest hormones play a supporting role, but sun exposure and genetics remain far more important.
Why Moles Exist at All
Moles don’t appear to serve any protective or adaptive purpose. They’re a byproduct of having melanocytes that can mutate when exposed to UV radiation or during normal cell division. From an evolutionary standpoint, they fall into the large category of things the body produces that are neither helpful nor harmful enough to be selected against. Because the body’s tissue-size feedback mechanisms almost always stop mole growth before it becomes dangerous, moles rarely affect survival or reproduction, so there’s been no evolutionary pressure to eliminate the tendency to form them.
When a Mole Deserves Attention
The vast majority of moles are benign, but because they involve the same type of cell and sometimes the same mutations found in melanoma, it’s worth knowing what a concerning change looks like. Dermatologists use five features, often called the ABCDE criteria:
- Asymmetry: one half of the mole doesn’t match the other
- Border irregularity: edges that are ragged, notched, or blurred rather than smooth
- Color variation: multiple shades of brown, black, tan, or patches of white, red, pink, or blue within the same mole
- Diameter: growth to larger than about 6 millimeters (roughly the size of a pencil eraser), though melanomas can be smaller
- Evolving: any noticeable change in size, shape, or color over weeks or months
Having many moles (50 or more) is itself a risk factor for melanoma, and people with certain hereditary mole patterns can have more than 100. If you have a high mole count or a family history of melanoma, regular skin checks become more important. A single new mole in adulthood is usually nothing to worry about, but a mole that’s actively changing in an asymmetric or multicolored way is worth having evaluated.