Babies birthmark or something else?! (Pic)

Cassandra • 26 years old. First pregnancy and I can't wait till Jan. 19th!
This is smack dab on top of my sons head. Wasn't this dark at 1st dr. appt with the pediatrician and he doesn't go back till March 3rd. She said it's probably a birth mark but that was said when it was very faint pink. Should I call to get in earlier? It doesn't appear painful to him or anything. Has any seen anything like this with their newborn?!