My 5 month old has a soft large lump underneath a birth mark on her hairline at back of head/neck . what is it ?

Hemangioma?: It could be a hemangioma. A hemangioma is an abnormal buildup of blood vessels in the skin or internal organs. Is is getting bigger, smaller, or staying about the same size as your baby grows? You should definitely have your child's doctor examine this at her next visit so that your doctor can track it over time. Most hemangiomas do not require treatment, though some do.