Forehead lines on babies are usually just from their facial expressions and the skin being so thin and soft at that age. My girl had the same thing when she was a newborn, and they faded on their own as she grew and her skin filled out more leh. It could also just be from how they were positioned in the womb, so there is not much to worry about. That said, if the lines look deep or you notice other things that concern you, a quick check with your PD will put your mind at ease.