A. A brown-colored spot anywhere on the skin that's getting larger is always something to be concerned about. Most brown spots on the skin turn out to be nothing serious, but some are melanoma--a potentially fatal skin cancer.
Asymmetry: Does one side or half of the lesion look different from the other half?
Border irregularities: Does it have an irregular border?
Color variation: Instead of being a consistent brown color, does it have some brown, some black, some red, some gray, and some white parts?
Diameter: Is it 6 millimeters or more across?
Evolving: Is it changing in size or color or shape?
The more of these five signs that such a spot has, the more worrisome it is. You've said it's getting larger (it meets the E criteria), so that's one reason to worry. I'd have your doctor examine the spot, or send you to a skin cancer specialist. If the spot has any of the other ABCD danger signs, it's even more important to have it checked out.
If you catch a melanoma at an early stage, it can be removed before it has a chance to spread. -- Anthony L. Komaroff, M.D., Editor in Chief, Harvard Health Letter