A sunken fontanel occurs when the soft spot on a baby's skull becomes more deep set than usual. These sutures give the skull some flexibility, allowing the head to pass through the birth canal. The anterior fontanel should curve slightly inwards but remain relatively firm.
Arq Neuropsiquiatr. 2007 Mar;65(1):170-2. Dermoid cyst of the anterior fontanelle in adults: case report. Castro RA(1), Ribeiro Filho Ade S, Silva VV Jr.
In newborns, soft spots are found on the top, back, and sides of the head. The number of soft spots on your baby’s head depends on their age. A soft spot with a noticeable inward curve is known as a sunken fontanel.
Sunken fontanelles are an obvious curving in of the "soft spot" in an infant's head.
Sunken. A sunken (also called "depressed") fontanelle indicates dehydration or malnutrition. Baby anterior fontanelle 1 month.