Sunken fontanelles are an obvious curving in of the "soft spot" in an infant's head.
Sunken fontanelles; Soft spot - sunken
The skull is made up of many bones. There are seven bones in the skull itself and 14 bones in the face area. They join together to form a solid, bony cavity that protects and supports the brain. The areas where the bones join together are called the sutures.
The bones are not joined together firmly at birth. This allows the head to change shape to help it pass through the birth canal. The sutures gradually gain minerals and harden, firmly joining the skull bones together. This process is called ossification.
In an infant, the space where two sutures join forms a membrane-covered "soft spot" called a fontanelle (fontanel). The fontanelles allow the brain and skull to grow during an infant's first year.
There are normally several fontanelles on a newborn's skull. They are located mainly at the top, back, and sides of the head. Like the sutures, fontanelles harden over time and become closed, solid, bony areas.
The fontanelle in the back of the head (posterior fontanelle) most often closes by the time an infant is 1 or 2 months old.
The fontanelle at the top of the head (anterior fontanelle) most often closes within 7 to 19 months.
The fontanelles should feel firm and should curve inward slightly to the touch. A noticeably sunken fontanelle is a sign that the infant does not have enough fluid in his or her body.
You might be referred to a place that can provide intravenous (IV) fluids if the sunken fontanelle is caused by dehydration.
Carlo W. The newborn infant. In: Kliegman RM, Behrman RE, Jenson HB, Stanton BF, eds. Nelson Textbook of Pediatrics. 19th ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 88.
Neil K. Kaneshiro, MD, MHA, Clinical Assistant Professor of Pediatrics, University of Washington School of Medicine, Seattle, WA. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.