Cranial sutures are fibrous joints that connect the bony plates of an infant’s skull. Infants’ skulls consist of several separate cranial bones, including two frontal, two parietal, and one occipital bone, which are held together by these strong, fibrous tissues. The spaces where multiple sutures intersect are known as fontanelles, often referred to as “soft spots.”
Why Sutures Exist
The presence of open cranial sutures and fontanelles in an infant’s skull serves two primary biological functions. During the birthing process, these fibrous joints allow the skull bones to overlap or “mold.” This flexibility enables the baby’s head to narrow and change shape, facilitating its passage through the relatively narrow birth canal without causing pressure or damage to the delicate brain.
Beyond birth, sutures and fontanelles accommodate the rapid growth of the brain during infancy and early childhood. The human brain undergoes significant expansion, reaching approximately 50% of its adult size by one year of age and continuing to grow until around 20 years. The flexible sutures act as expansion joints, allowing the skull to enlarge evenly as the brain increases in volume. Without these flexible connections, the brain would lack sufficient space to grow, which could lead to developmental issues.
Typical Closure Ages
The closure of cranial sutures and fontanelles involves the fibrous tissue turning into bone, a process called ossification. The metopic suture, located in the middle of the forehead between the two frontal bone plates, is the first to fuse. This suture begins to close around 3 to 4 months of age and is complete by 8 to 9 months.
The posterior fontanelle, a small triangular soft spot found at the back of the head where the two parietal bones meet the occipital bone, is the first fontanelle to close. This closure occurs early in infancy, often within 6 to 8 weeks after birth, or by 1 to 2 months of age.
The larger, diamond-shaped anterior fontanelle is located at the top-front of the head, where the frontal and parietal bones meet. This fontanelle remains open for a longer period to accommodate significant brain growth. It closes between 7 and 19 months of age, with an average closure around 12 to 14 months. Most anterior fontanelles are closed by 18 months.
Other sutures, such as the coronal, sagittal, and lambdoid sutures, close much later in life. The sagittal suture, running from the front to the back of the head between the two parietal bones, begins to close around 22 years of age, with complete fusion often occurring between 30 and 40 years. The coronal sutures, extending from ear to ear and separating the frontal and parietal bones, begin to fuse around 24 years of age, with complete closure noted between 30 and 40 years. The lambdoid sutures, located across the back of the head where the parietal bones meet the occipital bone, close around 26 years of age or between 30 and 40 years. These are average timelines, and individual variations in closure times are common.
What Happens When Closure Is Abnormal
Deviations from the typical closure timeline of cranial sutures and fontanelles can indicate underlying medical conditions. One such deviation is premature closure, known as craniosynostosis. This condition occurs when one or more sutures fuse too early, either before birth or within the first few months of life. Because the skull cannot expand perpendicular to the prematurely fused suture, brain growth can be restricted in that area, leading to compensatory growth in other directions. This results in an abnormally shaped head.
Craniosynostosis can affect various sutures; for example, premature closure of the metopic suture leads to a triangular forehead, while early fusion of the sagittal suture can result in a long, narrow head. If left uncorrected, craniosynostosis can lead to increased pressure inside the skull, which may affect brain development, causing developmental delays, headaches, or visual impairment. Medical intervention, typically surgery, is necessary to reshape the skull and allow adequate space for the growing brain.
Conversely, delayed closure occurs when sutures or fontanelles remain open longer than expected, which can signal a medical condition. For instance, a large or late-closing anterior fontanelle can be associated with conditions such as achondroplasia, a genetic disorder affecting bone growth, or hypothyroidism, which involves underactive thyroid function. Other causes include Down syndrome, rickets, or increased intracranial pressure. A sunken fontanelle indicates dehydration, while a bulging fontanelle suggests increased pressure within the brain. Medical evaluation by a pediatrician is advised if any concerns about fontanelle or suture closure are observed, to allow for assessment and diagnosis of any underlying issues.