What Causes Dark Spots On Hand During Pregnancy?

Hi i've dark spots in my left hand. It was a light color rash without any burning or itching and now it turned brown. I thought it was drying off but its been almost two weeks and its still there. What could it be? I'm pregnant and I asked my doctor but he said just to let it go away by itself but its been a 1 1/2 weeks since that and it's still there.