If present, the physician should rule out serious conditions associated with the third fontanelle. Other animals
Primates
In apes the fontanelles fuse soon after birth. In chimpanzees the anterior fontanelle is fully closed by 3 months of age. Dogs
One of the more serious problems that can affect canines is known as an "open fontanelle", which occurs when the skull bones at the top of the head fail to close. The problem is often found in conjunction with hydrocephalus, which is a condition in which too much fluid is found within and around the brain, placing pressure on the brain and surrounding tissues. Often the head will appear dome-shaped, and the open fontanelle is noticeable as a "soft spot" on the top of the dogs head. The fluid-filled spaces within the brain, known as ventricles, also become swollen. The increased pressure damages or prevents the development of brain tissue.Not all open fontanelles are connected with hydrocephalus. In many young dogs the skull bones are not fused at birth, but instead will close slowly over a three- to six-month period. Occasionally these bones fail to close, but the dog is still healthy. In these cases, however, the dogs owners need to be very careful, since any injury or bumps to the animals head could cause significant brain damage, as well as conditions like epilepsy. An open fontanelle, known as a "molera", is a recognized feature of the Chihuahua breed.
It is assumed that the term spring is used because of the analogy of the dent in a rock or earth where a spring arises.Parents may worry that their infant may be more prone to injury at the fontanelles. In fact, although they may colloquially be called "soft-spots", the membrane covering the fontanelles is extremely tough and difficult to penetrate.Fontanelles allow the infant brain to be imaged using ultrasonography. Once they are closed, most of the brain is inaccessible to ultrasound imaging, because the bony skull presents an acoustic barrier. Disorders
Bulging
A very tense or bulging anterior fontanelle indicates raised intracranial pressure. Increased cranial pressure in infants may cause the fontanelles to bulge or the head to begin to enlarge abnormally. It can occur due to:
Craniosynostosis – premature fusion of the cranial sutures
Encephalitis – swelling (inflammation) of the brain, most often due to infections
Hydrocephalus – a buildup of fluid inside the skull
Meningitis – infection of the membranes covering the brain
Shaken baby syndrome

Sunken
A sunken (also called "depressed") fontanelle indicates dehydration or malnutrition. Enlarged
The fontanelles may be enlarged, may be slow to close, or may never close, most commonly due to causes like:
Down syndrome
Hydrocephalus
Intrauterine growth restriction (IUGR)
Premature birthRarer causes include:
Achondroplasia
Apert syndrome
Cleidocranial dysostosis
Congenital rubella
Neonatal hypothyroidism
Osteogenesis imperfecta
Rickets

Third
Sometimes there is a third bigger fontanelle other than posterior and anterior ones in a newborn. In one study, the frequency of third fontanelles in an unselected population of newborn infants was 6.3%. It is very common in Down syndrome and some congenital infections.