During birth, a baby has a membranous gap between the bones of its skull. The bones have not yet joined together to ossify, or harden, like the rest of this part of the skeletal system. This gap is known as the fontanelle or a baby's "soft spot," and there are a few types of this anatomical feature. The biggest type is the anterior fontanelle, which is also known as the frontal fontanelle or bregmatic fontanelle.
The anterior fontanelle is described as a gap with the shape of a diamond or lozenge that measures around 1.6 inches (4 cm) by 1 inch (2.5 cm). It is placed between three types of connective tissue joints: the sagittal suture, coronal suture and frontal suture. The anterior fontanelle is sometimes called the bregmatic fontanelle because it contains the bregma, which is the point in the skull where the sagittal suture perpendicularly intersects the coronal suture.
The sagittal suture functions as the joint between the parietal bones, which are the pair of bones that form the sides and roof of the skull. By contrast, the coronal suture is the connective tissue joint responsible for setting the parietal bones apart from the cockleshell-shaped frontal bone, which comprises the forehead. The third connective tissue joint that forms the junction where the anterior fontanelle is located, the frontal suture, is named for its role of forming a temporary dividing line at the frontal bone. Thus, the anterior fontanelle encompasses areas of the frontal and parietal bones.
Fontanelles, in general, exist so that the skull can bend to make it easier for the baby to pass through the birth canal. Even after birth, they remain to allow the young brain to expand. The anterior fontanelle is so named because it appears at the front of the skull. Other types of fontanelles include the posterior fontanelle, which is found at the back of the cranium—specifically, where the two parietal bones meet with the skull's bottom rear part, known as the occipital bone. Another type of fontanelle, the lateral fontanelle, forms at the sides of the skull.
The posterior and lateral fontanelles disappear within six months of birth. The anterior fontanelle, however, does not close until the conclusion of a person's second year of life. Moreover, it does not begin to ossify until the person approaches the age of 30. The hardening process concludes by the age of 50.