Fontanelles, commonly known as soft spots, are natural features of an infant’s skull that allow for safe passage during birth and accommodate rapid brain growth after delivery. The newborn skull is composed of several bone plates separated by flexible fibrous joints called sutures. These open spaces permit the necessary molding and slight overlap of the skull bones as the baby moves through the narrow birth canal. The posterior fontanelle is the focus of this discussion, as its early closure marks one of the first developmental milestones of the head structure.
Anatomy and Location of the Posterior Fontanelle
The posterior fontanelle is found at the back of the infant’s head, specifically where the two parietal bones meet the single occipital bone. This location corresponds to the junction of the sagittal and lambdoid sutures, which are the fibrous seams separating the skull plates. Unlike the larger, diamond-shaped soft spot on the top of the head, the posterior fontanelle is distinctly triangular in shape. This fontanelle is significantly smaller than its anterior counterpart, typically measuring between 0.5 and 0.7 centimeters at birth. Its presence is just as important for cranial flexibility in the newborn period. The adult anatomical landmark that replaces the posterior fontanelle after closure is known as the lambda.
The Expected Timeline for Closure
The posterior fontanelle is the first of the major fontanelles to close, which typically happens much sooner than parents might expect. The normal window for closure spans from approximately six weeks to three months of age. In some instances, this soft spot may already be closed or nearly closed at the time of birth, which can still be within the normal range of development. This relatively quick closure is a result of the slower rate of brain growth in the occipital region compared to the frontal and parietal areas in the first few months of life. Ossification, the process where the fibrous membrane is replaced by bone tissue, begins rapidly in this area shortly after birth. This early milestone confirms that the skull is maturing appropriately to provide protection to the underlying brain tissue.
What Variations in Closure Timing May Indicate
Early Closure
A deviation from the typical 6-week to 3-month window can sometimes signal an underlying medical condition, though an isolated variation is often benign. If the posterior fontanelle closes before six weeks, it is considered early closure, which warrants careful monitoring of the infant’s head circumference. Premature closure may be a sign of craniosynostosis, a condition where one or more of the skull sutures fuse too soon. This premature fusion can restrict normal brain growth and lead to an abnormally shaped head, requiring potential intervention. Early closure can also be associated with microcephaly, a condition where the head is smaller than average due to abnormal brain development.
Delayed Closure
Conversely, a posterior fontanelle that remains open after three months is classified as delayed closure. This finding is often linked to systemic conditions affecting bone development or metabolism, and requires further investigation by a pediatrician. The most common conditions associated with late closure include:
- Congenital hypothyroidism, which is a lack of thyroid hormone that affects overall growth and development.
- Hydrocephalus, which involves a buildup of fluid that increases pressure inside the brain.
- Rickets, a bone-softening disease caused by vitamin D deficiency.
- Certain genetic syndromes, such as Down syndrome.
Parents should consult a pediatrician if the posterior soft spot feels rigid before six weeks or if it is still noticeably open after three months, ensuring appropriate developmental tracking.