The anterior fontanelle (bregmatic fontanelle, frontal fontanelle) is the largest fontanelle

Fontanelle

A fontanelle is an anatomical feature on an infant's skull.-Anatomy:Fontanelles are soft spots on a baby's head which, during birth, enable the bony plates of the skull to flex, allowing the child's head to pass through the birth canal. The ossification of the bones of the skull causes the...

, and is placed at the junction of the sagittal suture

Sagittal suture

The sagittal suture is a dense, fibrous connective tissue joint between the two parietal bones of the skull. The term is derived from the Latin word Sagitta, meaning "arrow". The derivation of this term may be demonstrated by observing how the sagittal suture is notched posteriorly, like an arrow,...

, coronal suture

Coronal suture

The coronal suture is a dense, fibrous connective tissue joint that separates the frontal and parietal bones of the skull. At birth, the bones of the skull do not meet.-Pathology:...

, and frontal suture

Frontal suture

The frontal suture is a dense connective tissue structure that divides the two halves of the frontal bone of the skull in infants and children. It usually disappears by the age of six, with the two halves of the frontal bone being fused together...

; it is lozenge

Lozenge

A lozenge , often referred to as a diamond, is a form of rhombus. The definition of lozenge is not strictly fixed, and it is sometimes used simply as a synonym for rhombus. Most often, though, lozenge refers to a thin rhombus—a rhombus with acute angles of 45°...

-shaped, and measures about 4 cm in its antero-posterior and 2.5 cm in its transverse diameter. The fontanelle allows the skull to deform during birth to ease its passage through the birth canal and for expansion of the brain after birth.

While the posterior and lateral fontanelles are obliterated by about six months after birth, the anterior is not completely closed until about the middle of the second year. Full ossification starts in the late twenties and finishes before the age of 50 .

Clinical significance

The anterior fontanelle is useful clinically. Examination of an infant includes palpating the anterior fontanelle. A sunken fontanelle indicates dehydration

Dehydration

In physiology and medicine, dehydration is defined as the excessive loss of body fluid. It is literally the removal of water from an object; however, in physiological terms, it entails a deficiency of fluid within an organism...

, whereas a very tense or bulging anterior fontanelle indicates raised intracranial pressure

Intracranial pressure

Intracranial pressure is the pressure inside the skull and thus in the brain tissue and cerebrospinal fluid . The body has various mechanisms by which it keeps the ICP stable, with CSF pressures varying by about 1 mmHg in normal adults through shifts in production and absorption of CSF...

. However, this is not a certain indicator for raised pressure as prolonged crying by the baby may produce the same effect.