Red creases on the neck

My son had this too and it's usually just trapped sweat and milk in those folds, so keeping the area clean and dry is key. I'd wipe it gently after feeds and baths, then pat it completely dry before putting him down, and the redness cleared up pretty quickly once I got into that habit. If it starts looking really irritated or doesn't improve in a few days, might be good to bring him to see the PD just to rule out anything else, but most of the time it's just about moisture control.