Erythema Toxicum Neonatorum (ETN): This is a common, benign condition that appears as a rash in newborns, often within the first few days of life. It can present behind the ears, among other areas, and is characterized by small, yellow or white bumps on a red base. The condition is self-limiting and resolves without treatment.

Milialia (Baby Acne): While more commonly found on the face, milialia can occasionally appear behind the ears. It's caused by the blockage of oil glands and is also benign, resolving on its own.
Seborrhoeic Dermatitis (Cradle Cap): This condition can cause a rash that might appear behind the ears, though it's more commonly associated with the scalp. It's characterized by a greasy, yellow, scaly rash.
Contact Dermatitis: Newborns can develop a rash due to irritation from soaps, lotions, or other products used on their skin. This could potentially occur behind the ears if these products are used in that area.

Congenital Infections (e.g., Congenital Syphilis, Rubella): Although less common, certain congenital infections can cause rashes in newborns. These conditions are critical to identify early due to their potential for serious complications.
Bacterial or Fungal Infections: Infections such as impetigo or candidiasis can cause rashes and need to be treated promptly to prevent spread and complications.

Incontinentia Pigmenti: A rare genetic disorder that can cause skin lesions, including behind the ears, among other systemic manifestations.
Neonatal Lupus: A rare condition caused by the transplacental transfer of autoantibodies from mother to fetus, which can result in a rash, among other symptoms.
Langerhans Cell Histiocytosis: A rare disorder in which the body accumulates too many immature Langerhans cells, leading to a variety of symptoms, including skin rashes.