Newborn not having a very swell time

The Case

You are the senior resident in charge of the general pediatrics team. Just as you finish rounding, the emergency department pages you about a 1-month-old girl recently transferred from an outside hospital for further evaluation of swelling over her posterior scalp.

The patient's mother first noticed the swelling, which she describes as "like water," 2 days ago. There is no ecchymosis, overlying skin changes, or drainage from the area. The patient is otherwise well, displays no unusual movements or behavior, and feeds without difficulty. The mother denies recent trauma or any other inciting event.

She received a vitamin K injection at birth and hepatitis B vaccination before discharge from the nursery. You call her primary pediatrician and learn that she had a normal scalp exam immediately postpartum as well as on days of life 4 and 14.

The patient lives at home with both parents who immigrated to the United States from Central America 3 years ago. The child is cared for at home by her mother and has no recent travel or exposure to visitors.

The mother's medical history is positive for menorrhagia, but the family history otherwise is negative, including no history of bleeding, skeletal, or neurologic diseases.

On physical exam, you find the patient to be awake, alert, and in no acute distress. You note that the recorded weight, length, and head circumference are appropriate for her age.

The head and neck exam shows a 4 cm × 4 cm area of soft swelling that crosses the midline over her occipital scalp. There is no apparent tenderness or overlying skin changes in the area. Otherwise the exam is unremarkable.