In newborns and infants, the soft areas where adjacent bones meet and that are filled with connective tissue are much broader. They represent the sites where the ossification has not started yet. These soft membranous areas are especially wide at the top and sides of the skull. Here, they are known as fontanelles.

During birth, fontanelles allow the skull bones to overlap slightly, thus providing flexibility and allowing the head of the infant to go through the birth canal. Moreover, fontanelles enable the brain and skull to grow and enlarge rapidly after birth.

Each newborn has six fontanelles - two major and four minor fontanelles. Each major fontanelle is a single structure, while the minor fontanelles are paired. The major fontanelles are large, and they are found at the top of the skull. In contrast, the minor fontanelles are smaller and situated on the sides of the skull.

The major fontanelles are the frontal and occipital fontanelles:

The frontal fontanelle, also known as the anterior or major fontanelle, is found at the junction of the coronal and sagittal sutures. It usually closes between (12) 18 to 24 months.
The occipital fontanelle, also called the posterior or minor fontanelle, is located at the junction between the sagittal and lambdoid sutures. It typically closes between the second and third months of age.

The minor fontanelles include the sphenoid and mastoid fontanelles:

The sphenoid fontanelle, also known as the anterolateral fontanelle, is located between the sphenoid, temporal, frontal, and parietal bones. It closes around six months after birth.
The mastoid fontanelle, or the posterolateral fontanelle, is situated between the temporal, occipital, and parietal bones. It closes from 6 to 18 months after birth.

During embryological development and childhood, sutures function as intramembranous bone growth sites and only fuse and ossify later in life. New bone is produced at the sutural edges as a result of coordinated external stimuli. Delayed or restricted bone growth results in suture agenesis and wide-open fontanelles. Moreover, excessive bone growth can also result in deformities of the skull.

During the first year after birth, the fontanelles significantly narrow as the skull bones expand. When the connective tissue between the adjacent bones diminishes to a thin layer, these fibrous joints are referred to as sutures. In some areas, the connective tissue will ossify and transform into bone, leading to the fusion of the adjacent bones.

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Fontanelles