Caput succedaneum: This is a common condition in newborns, especially after vaginal delivery, where the scalp swells due to pressure exerted during birth. It typically resolves on its own within a few days but can be present for up to a month.

Cephalohematoma: A collection of blood between the skull and its covering, often resulting from birth trauma. It usually appears a few days after birth and can take weeks to resolve.
Soft tissue injury or bruise: A bump could result from a minor injury, such as a fall or accidental bump against an object, which is common in infants learning to navigate their environment.
Congenital cysts or benign tumors: Though less common, conditions like dermoid cysts, epidermoid cysts, or other benign growths could present as a bump on the back of the head.

Meningocele or Myelomeningocele: These are forms of spina bifida where the meninges and spinal cord protrude through an opening in the spine. Early detection is crucial for surgical intervention to prevent infection and further neurological damage.
Subgaleal hemorrhage: A serious condition where blood collects between the skull and the scalp, potentially leading to significant blood loss and shock. It requires immediate medical attention.
Osteomyelitis or skull infection: Though rare, an infection of the bone or skull can present as a bump and is a medical emergency due to the risk of serious complications, including abscess formation and spread of infection.

Langerhans cell histiocytosis: A rare disorder where the body accumulates too many immune cells called Langerhans cells, which can lead to various symptoms, including bumps on the skull.
Craniopharyngioma: A rare type of brain tumor that could potentially cause a bump if it grows large enough to exert pressure on the skull from the inside.
Neurofibromatosis: A genetic disorder that can cause tumors to form on nerve tissue, potentially leading to bumps on the head, though this would be part of a broader spectrum of symptoms.