I have something like a large birthmark on one of my upper thigh covering half of my buttocks and a little of my lower back on the same side. I think it appeared some time during puberty. My mother told me it wasn't there when I was born. It's not itchy or painful, and it's not spreading anymore, but it is very large and noticeable. Although sometimes my skin gets itchy and I sometimes scratch myself, I don't think scratching could have caused this. One of my upper thigh and buttock and lower back are the only affected places. What is it? Please help me find out the name of this condition. And what can I do about it? Thank you.