A fontanelle is a soft spot on a baby’s skull where the bones haven’t yet fused together. Newborns have six of these gaps, covered by a tough membrane rather than bone. They serve two critical purposes: allowing the skull to compress slightly during birth so the baby can fit through the birth canal, and giving the brain room to grow rapidly during the first two years of life.
Where the Six Fontanelles Are
The one most parents notice is the anterior fontanelle, the diamond-shaped soft spot on top of the head toward the front. It sits where the two frontal bones and two parietal bones meet, and it’s the largest of the six. The posterior fontanelle is smaller and triangle-shaped, located at the back of the skull where the parietal bones meet the occipital bone.
The remaining four are paired, one on each side of the head. The sphenoid fontanelles (also called anterolateral fontanelles) sit on either side of the skull near the temples, where the sphenoid, parietal, temporal, and frontal bones converge. The mastoid fontanelles (posterolateral fontanelles) are farther back, at the intersection of the temporal, parietal, and occipital bones. These side fontanelles are much smaller and harder to feel than the two along the midline.
When Each Fontanelle Closes
The posterior fontanelle is the first to close, typically within the first two to three months after birth. The sphenoid fontanelles close next, around six months. The mastoid fontanelles follow on a wider timeline, closing anywhere from 6 to 18 months.
The anterior fontanelle stays open the longest because it covers the area of most rapid brain growth. Research using modern imaging puts typical closure between 14 and 22 months, with some variation by sex and ethnicity. One study found that in males, the anterior fontanelle closes between roughly 14 and 19 months, while in females the range extends to about 16 to 22.5 months. A separate study of New Zealand infants found that Maori and Pacific Islander babies had the anterior fontanelle closed by 19 months, while half of European-descent babies in the same age group still had remnants of the opening at 19 to 24 months. In other words, there’s a wide range of normal.
What a Normal Fontanelle Feels Like
When you gently touch your baby’s anterior fontanelle, it should feel soft and flat, roughly level with the surrounding skull. You may notice it pulsing slightly, which is normal. That pulse reflects blood flow through the vessels near the brain’s surface. The fontanelle may also bulge briefly when a baby cries, strains, or vomits, then return to its flat resting position. This temporary bulging during crying is not a concern.
The membrane covering a fontanelle is surprisingly sturdy. It’s made of dense connective tissue, not just skin. You won’t hurt your baby by gently washing their scalp, brushing soft hair over the area, or lightly touching it during everyday care.
What a Sunken Fontanelle Means
If the anterior fontanelle dips noticeably inward, creating a visible dip or hollow, it’s often a sign of dehydration. This is especially common when a baby has been vomiting, having diarrhea, or not feeding well due to illness. The NHS lists a sunken fontanelle as a sign of serious dehydration requiring urgent medical attention. If you notice this alongside fewer wet diapers, dry lips, or unusual drowsiness, your baby needs fluids and likely a prompt medical evaluation.
What a Bulging Fontanelle Means
A fontanelle that stays tense, raised, or firm even when your baby is calm and upright is a different kind of warning sign. A persistently bulging fontanelle occurs when fluid builds up in the brain or the brain swells, causing increased pressure inside the skull. The main conditions that cause this include meningitis (infection of the membranes around the brain), encephalitis (swelling of the brain itself, usually from infection), and hydrocephalus (a buildup of fluid within the skull).
A truly bulging fontanelle, especially combined with fever, excessive sleepiness, poor feeding, or irritability, is a medical emergency. The fontanelle may feel taut, almost as hard as bone. This is distinct from the brief, temporary bulging that happens during crying.
Delayed or Early Closure
Sometimes a fontanelle closes earlier or later than expected, which can signal an underlying condition. Early closure (called craniosynostosis) happens when skull bones fuse prematurely, potentially restricting brain growth. This is usually detected during routine pediatric checkups when a doctor measures head circumference and checks the fontanelle.
A fontanelle that remains unusually large or stays open well past the expected timeline can be associated with a range of conditions, including certain genetic and metabolic disorders that affect bone development. Your baby’s doctor monitors the fontanelle at every well-child visit for exactly this reason, tracking both its size and how it’s progressing toward closure over time.