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.
Examination of an infant includes palpating the anterior fontanelle. Two smaller fontanelles are located on each side of the head, more anteriorly the sphenoidal or anterolateral fontanelle (between the sphenoid, parietal, temporal, and frontal bones) and more posteriorly the mastoid or posterolateral fontanelle (between the temporal, occipital, and parietal bones).During birth, fontanelles enable the bony plates of the skull to flex, allowing the childs head to pass through the birth canal. The ossification of the bones of the skull causes the anterior fontanelle to close over by 9 to 18 months. The sphenoidal and posterior fontanelles close during the first few months of life. The closures eventually form the sutures of the neurocranium. Other than the anterior and posterior fontanelles, the mastoid fontanelle and the sphenoidal fontanelle are also significant. Closure
In humans, the sequence of fontanelle closure is as follows:
The posterior fontanelle generally closes 2 to 3 months after birth;
The sphenoidal fontanelle is the next to close around 6 months after birth;
The mastoid fontanelle closes next from 6 to 18 months after birth; and
The anterior fontanelle is generally the last to close between 12 and 18 months. Clinical significance
The fontanelle may pulsate, and although the precise cause of this is not known, it is normal and seems to echo the heartbeat, perhaps via the arterial pulse within the brain vasculature, or in the meninges. This pulsating action is how the soft spot got its name – fontanelle is borrowed from the old French word fontenele, which is a diminutive of fontaine, meaning "spring".