Baby Soft Spot: Why Babies Have Fontanelles and When They Close

A newborn's skull is not one solid bowl. It is made of separate bone plates joined by flexible, fibrous seams called sutures. Where those seams meet, wider spaces covered by membrane remain, and those are the fontanelles, or soft spots.

The anterior fontanelle is the one most parents notice. It is the larger, diamond-shaped soft spot at the top front of the head, where the frontal bone (the forehead) meets the two parietal bones (the sides and top of the skull). The posterior fontanelle is smaller and triangular, and sits at the back of the head, where the parietal bones meet the occipital bone at the base of the skull, according to Stanford Medicine Children's Health. Smaller fontanelles can also occur at the sides of the skull. Many families call the front soft spot "la mollera."

Your baby's doctor or nurse looks at the fontanelles and sutures during routine checkups. Their shape, firmness and closure help track how the skull and brain are growing, as MedlinePlus explains.

Soft spots do two jobs, one at birth and one for many months afterwards.

During birth, the skull bones can shift and overlap slightly, which lets the baby's head pass through the birth canal. After birth, the open spaces and flexible sutures let the skull expand as the brain grows rapidly through infancy.

The fontanelles are not open holes into the brain. The Mayo Clinic and Stanford Medicine Children's Health both describe them as covered by a strong, fibrous membrane. For a skull lover, it is a neat piece of design: a head built to be flexible first and solid later.

The two main soft spots do not close at the same time.

The posterior fontanelle, at the back, usually closes first. MedlinePlus gives a typical range of about 1 to 3 months, and in some babies it is already closed at birth. The anterior fontanelle, at the front, closes later. Medical guides give several commonly cited ranges: about 7 to 19 months on MedlinePlus, 9 to 18 months from Nationwide Children's Hospital, and 12 to 24 months in other guidance. The Mayo Clinic describes closure by about age two.

Those ranges overlap but do not match, and that is the point: individual variation is normal. A child's doctor does not judge the soft spot by the calendar alone. They look at it together with head growth, the shape of the skull and the child's overall development.

Even after the front soft spot closes, the skull keeps growing. The sutures remain visible and continue to change through childhood, so there is no single day when a baby's head becomes "fully hard."

A relaxed, healthy fontanelle is generally flat or slightly curved inward. It can feel soft or fairly firm. You may also see it move gently in time with your baby's heartbeat, which the American Academy of Pediatrics' HealthyChildren.org describes as commonly normal.

If your fingers brushed the soft spot during a bath, that is a very common question. Gentle, routine touching while holding your baby, washing their hair or examining their head is generally safe, because the strong membrane covers the space, according to the Mayo Clinic and HealthyChildren.org.

A real knock or fall is a different matter. If your baby has a significant bump to the head, or afterwards develops a bulging soft spot, vomiting, unusual sleepiness, a change in behavior or any other symptom that worries you, call your child's doctor or seek urgent care.

Most of the time, a soft spot is just a soft spot. A few changes, though, are worth a call.

A sunken soft spot. A mildly indented fontanelle can be normal. A noticeably sunken one can go with dehydration, especially alongside poor feeding, a dry mouth, fewer wet diapers, unusual sleepiness or fussiness, according to MedlinePlus. Call your child's doctor.

A bulging soft spot. A fontanelle that stays bulging or swollen, especially while the baby is calm and not crying, can be a sign of increased pressure inside the skull or another serious problem, as MedlinePlus and the Cleveland Clinic explain. Call promptly. If the bulge comes with vomiting, unusual sleepiness, a change in behavior, fever or a head injury, get urgent medical care.

A head shape that looks unusual. When one or more sutures close too early, the condition is called craniosynostosis. Signs can include an unusual head shape, a hard ridge along one of the seams, or changes in the symmetry of the forehead, ears or eyes, according to the Mayo Clinic. A soft spot that is hard to feel does not, on its own, mean a baby has craniosynostosis, which is why a doctor needs to assess it.

The simple rule from these children's health sites: if a change in your baby's soft spot or head shape concerns you, call the doctor, and if your baby seems ill, get urgent care.

An adult skull is conventionally counted as 22 bones, 8 cranial bones around the brain and 14 facial bones, according to OpenStax anatomy. A newborn's skull is not simply a smaller version of that. Its main plates are still separate and joined by sutures, with fontanelles at several junctions, and they have not yet grown together into the more continuous adult structure.

The whole skeleton tells a similar story. OpenStax states that babies have about 270 to 300 bones at birth, while adults conventionally have 206, because some of a baby's bones later fuse. That whole-body count is separate from the 22-bone count for the adult skull. If you want the full count bone by bone, our story on how many bones are in the human body has it, and human skull facts covers the adult skull in more detail.

A few more questions come up often.

A baby's soft spots are one of the quietly clever features of the human skull: flexible for birth, open for growth, and closed in their own time. Knowing what is normal makes it easier to stay calm, and knowing the few signs that matter makes it easy to know when to call. If you are curious about the skull your baby is growing into, how strong a human skull is and how much a human skull weighs pick up the story from there.

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