It is one sign of pregnancy, yes, but the only conclusive way would be to take a pregnancy test. If you have unprotected sex or think there is some other good reason you might be pregnant, it's worth finding out as soon as you can.
I've had a dark band around my nipples also, and it's getting wider. I was told it's a sign of high estrogen levels. I'm unable to get pregnant, but I can't say if it might be a sign of pregnancy in someone else.
