What is this pinkish red birthmark on my baby's forehead?

Likely a bening issu. Many newborns have benign "tumors" of blood vessels in the skin and a vast majority regress with age. If the "birthmark" is stable, there is no need to worry and you may wait to discuss it with the pediatrician at the next scheduled visit.
HARD TO SAY. Infants have a variety of "pink" birthmarks that can appear. The most benign often carry labels like angel kisses or rose spots. They can exist over the eyelids, mid brow and nose, and fade as the skin thickens. There is a worrisome "port wine stain" birthmark that shows up on the side of the temple or face. Discuss with babies doc. Without examining baby I cannot be sure.