Black and brown moles both result when skin cells grow together in a cluster rather than spreading throughout the skin, according to WebMD. The name for these cells is melanocytes, and they are the source of skin's natural pigmentation. Moles often darken as a result of solar exposure and pregnancy.
The majority of moles show up during childhood and a person's first 30 years of life. Having anywhere between 10 and 40 moles as an adult is normal. Time brings changes to moles, as colors shift and hairs sometimes appear. Some vanish altogether, while others always remain the same. Most moles do not present a danger. However, those with a greater likelihood of cancer look different than those around them or show up after the age of 30, notes WebMD.
Moles come in two types: congenital nevi and dysplastic nevi. Congenital moles are present from birth and happen in about 1 percent of the population. These are sometimes more likely to turn into melanoma than moles that appear later. If freckles or moles are larger than a pencil eraser in diameter, a dermatologist should examine them. Dysplastic moles are larger than the mean and have an anomalous, or irregular, shape and uneven color and borders. People who have at least 10 dysplastic moles are 12 times more likely to develop melanoma later in life, which means that these moles need regular checking. Any mole that bleeds, itches, oozes or hurts should also be analyzed, advises WebMD.