The skull of a newborn contains several soft, membranous gaps between the bone plates, known as fontanelles. These features are a necessary part of early development, serving two primary functions related to the rapid growth of the brain and the process of birth itself. The soft spots provide flexibility, allowing the bony plates to shift and overlap slightly during passage through the narrow birth canal. This temporary molding of the head protects the delicate brain tissue during delivery. Once the baby is born, the fontanelles continue to function as expansion joints, accommodating the massive increase in brain size that occurs in the first two years of life.
Anatomy and Purpose of Infant Soft Spots
Fontanelles are areas of fibrous connective tissue separating the cranial bones before they fully fuse into sutures. While a baby is born with six fontanelles, the two most prominent are the anterior and the posterior fontanelles. The anterior fontanelle, often referred to as the main soft spot, is the largest and is located on the top front of the head, roughly diamond-shaped.
The posterior fontanelle is positioned at the back of the head, situated at the meeting point of the two parietal bones and the occipital bone. This soft spot is notably smaller than its anterior counterpart and typically has a triangular shape. Both fontanelles are covered by a tough, protective membrane. Examining the tension of these spots during check-ups offers a quick, non-invasive way for doctors to assess the baby’s intracranial pressure.
Typical Timeline for Posterior Fontanelle Closure
The posterior fontanelle is the first of the major soft spots to close, a process that happens relatively quickly after birth. For most healthy, full-term infants, this fontanelle closes between one and three months of age. It is not uncommon for the posterior fontanelle to be small or even closed at the time of birth in some newborns.
This early fusion occurs through the ossification process, where the fibrous tissue is replaced by bone, completing the formation of the lambdoid and sagittal sutures at the back of the head. This rapid closure contrasts sharply with the anterior fontanelle, which remains open much longer, often not closing until sometime between seven and nineteen months of age.
What Deviations in Closure Timing May Indicate
The timing of fontanelle closure is an important indicator of a child’s overall health and developmental status. Pediatricians routinely monitor the size and feel of the fontanelles during check-ups to ensure the process is progressing normally. A fontanelle that closes too early or too late can signal an underlying medical condition affecting bone development or intracranial pressure.
Premature closure, particularly if it occurs much earlier than the one-month mark, may prompt concern for conditions like craniosynostosis. This is a condition where one or more of the fibrous sutures fuse too soon, which can restrict the normal growth and shape of the brain. In these cases, the pediatrician will closely track the infant’s head circumference to ensure the brain is still growing appropriately despite the early fusion.
Conversely, a posterior fontanelle that remains open past three months is considered delayed closure. This finding can be associated with various conditions, including metabolic disorders such as hypothyroidism or skeletal dysplasias. Delayed closure may also be linked to increased pressure inside the skull, which can be caused by conditions such as hydrocephalus, where excess fluid accumulates.
During physical examinations, a fontanelle that feels firm and flat is considered normal. A bulging or tense fontanelle, especially when the baby is calm and upright, can be a sign of increased intracranial pressure and requires immediate medical evaluation. Conversely, a significantly sunken fontanelle may indicate dehydration or malnutrition.