Posterior fontanelle

The posterior fontanelle is the small soft spot at the back of an infant’s skull where the parietal and occipital bones meet. In Anatomy and Physiology I, it shows how the skull develops and why infant cranial bones are not fully fused at birth.

What is the posterior fontanelle?

The posterior fontanelle is the small, triangular soft spot on the back of an infant’s skull. It sits where the parietal bones meet the occipital bone, so it is one of the places where the skull has not yet fully ossified.

In Anatomy and Physiology I, you usually meet this term while studying embryonic development of the axial skeleton and skull formation. The skull is not built as one solid structure from the start. Instead, several bones form separately and then fuse over time as the infant grows. Fontanelles are the gaps between these bones, and the posterior fontanelle is one of the first to close.

This early closure matters because the newborn skull needs some flexibility. During birth, the bones can shift slightly and overlap, which helps the head pass through the birth canal. After delivery, that same openness gives the brain room to grow rapidly during the first months of life.

The posterior fontanelle is much smaller than the anterior fontanelle and usually closes within 2 to 3 months after birth. Because it closes so early, it is often used as a sign that cranial development is proceeding on schedule. If it closes too late, stays unusually large, or feels abnormal on exam, that can point to a developmental or health concern that needs follow-up.

A common mistake is to think fontanelles are just random gaps in the skull. They are actually part of normal anatomy. They reflect the timing of intramembranous ossification in the skull vault and show that the infant skeleton is still changing fast after birth.

Why the posterior fontanelle matters in Anatomy and Physiology I

The posterior fontanelle gives you a clear example of how structure and function work together in human anatomy. It connects skull anatomy to birth mechanics, because the small gap lets the fetal head compress slightly during delivery and then re-expand afterward.

It also helps you see that the infant skull is not just a mini version of the adult skull. The timing of bone formation matters. The cranial vault has to stay flexible enough for birth and brain growth, which is why the fontanelle exists at all.

In A&P, this term often shows up when you are comparing skull bones, learning intramembranous ossification, or looking at the difference between normal growth and abnormal findings. A note about a fontanelle on an infant exam is not just a detail, it can be a clue about cranial development, hydration status, or a developmental disorder.

The term also supports later topics like cranial sutures and congenital conditions that affect skull shape. Once you understand the posterior fontanelle, the rest of early skull development makes more sense instead of feeling like a list of bone names.

How the posterior fontanelle connects across the course

Fontanelles

The posterior fontanelle is one of the fontanelles, the soft gaps between skull bones in an infant. Studying fontanelles as a group helps you compare their size, location, and closure timing. That comparison is useful because not all fontanelles close on the same schedule, and that timing shows how the skull matures after birth.

Cranial sutures

Cranial sutures are the fibrous joints that eventually replace the soft gaps as the skull bones fuse. The posterior fontanelle sits where nearby sutures meet, so it is part of the same developmental picture. Once the fontanelle closes, the sutures remain as flexible joints that still allow growth during infancy and childhood.

Anterior fontanelle

The anterior fontanelle is the larger, longer-lasting soft spot at the front of the infant skull. It is easy to confuse with the posterior fontanelle, but it closes much later and is much larger. Comparing the two helps you remember that different parts of the skull ossify at different rates.

Hydrocephalus

Hydrocephalus can cause the fontanelles to bulge because increased fluid pressure affects the still-open infant skull. That makes fontanelle assessment clinically useful, not just anatomical. If a case study mentions a tense or bulging soft spot, you may need to connect it to pressure inside the skull rather than normal development.

Is the posterior fontanelle on the Anatomy and Physiology I exam?

A quiz question may show an infant skull diagram and ask you to identify the posterior fontanelle by location, shape, or closure timing. You may also see a case question about an infant whose skull soft spot is still open, and you will need to decide whether that fits normal development or suggests a concern.

On lab practicals, this term often shows up in bone models, diagrams, or photos of the neonatal skull. The task is usually to recognize that the posterior fontanelle is the small soft spot at the back of the head where the parietal and occipital bones meet.

If the question asks why it exists, connect it to childbirth and early brain growth. If it asks what happens next, explain that the posterior fontanelle closes early as cranial bones fuse during infancy.

The posterior fontanelle vs Anterior fontanelle

These two are easy to mix up because both are soft spots in an infant’s skull. The posterior fontanelle is smaller, triangular, and located at the back of the head, while the anterior fontanelle is larger and found toward the front. They also close at different times, with the posterior fontanelle closing much earlier.

Key things to remember about the posterior fontanelle

• The posterior fontanelle is the small soft spot at the back of an infant’s skull where the parietal and occipital bones meet.

• It is part of normal infant skull development, not a defect, and it reflects incomplete ossification at birth.

• Its flexibility helps the skull compress slightly during childbirth and leaves room for rapid brain growth after birth.

• The posterior fontanelle is smaller than the anterior fontanelle and usually closes within 2 to 3 months.

• In Anatomy and Physiology I, changes in the fontanelle can help you connect skull anatomy to growth patterns and possible developmental concerns.

Frequently asked questions about the posterior fontanelle

What is posterior fontanelle in Anatomy and Physiology I?

The posterior fontanelle is the small, triangular soft spot on the back of an infant’s skull. It forms where the parietal and occipital bones meet. In A&P, it is a good example of how the infant skull stays flexible before the bones fully fuse.

How is the posterior fontanelle different from the anterior fontanelle?

The posterior fontanelle is smaller and closes much earlier, usually within the first few months after birth. The anterior fontanelle is larger and stays open longer. If you are looking at a skull image, location and size are the fastest clues.

Why does the posterior fontanelle exist?

It exists because the infant skull is still developing. The soft spot helps the head compress during childbirth and gives the brain space to grow rapidly after birth. That makes it a normal part of neonatal anatomy.

What does an abnormal posterior fontanelle mean?

An unusually large, delayed, or bulging posterior fontanelle can point to a developmental or medical issue. In a class case study, you would not diagnose from that alone, but you would recognize it as a sign worth checking against other findings.