Not Medical Advice: Possible causes of sunken soft spots on the head include dehydration, injury, abscess, cyst, or benign lipoma. A lipoma is a soft tissue benign tumor, usually painless, that is common in adults 40 to 60.
The skull is made up of many bones. There are 8 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.
To learn more, visit MedlinePlus.