What is this on my newborns arm ???

Juli Ann • Mommy of Emerson (02/26/18) Follow me on Instagram @julesmariea

My baby boy is almost 7 weeks and has had this on his arm for over a week. At first I though maybe I pinched him somehow but it’s been there for so long I’m kinda thinking it something else. It isn’t bumpy or anything. Do you guys have any ideas ?