Wow! I've had these skin patches under one armpit for years and they have come and gone dependant on my weight. I also got them across under my breasts and between my breasts when I was at my heaviest. They were much darker during pregnancy and when I was a lot heavier.

There you go! I never knew what they were. Sort of worries me a bit that they are related to diabetes, as my father has type II (adult onset) diabetes. I've always been very fit and healthy apart from a few years in my adult life - so I was kind of hoping that I wouldn't have to worry about diabetes.