Quick answer: Fontanelles are gaps between the skull bones covered by a tough membrane, allowing the head to flex during birth and accommodate rapid brain growth. The anterior (front) fontanelle closes at 12–18 months; the posterior (back) closes by 6–8 weeks.
Anatomy and function
Babies have two main fontanelles. The anterior fontanelle is diamond-shaped, approximately 2cm across at birth, felt as a slightly soft area at the top of the head. It closes between 12–24 months (average 18 months). The posterior fontanelle is smaller and triangular, located at the back of the head; it closes by 6–8 weeks. The pulsation you feel at the anterior fontanelle is arterial blood pressure transmitted through the membrane — completely normal. The skull bones themselves are separated by fibrous joints (sutures) that allow continued head growth — the fontanelles are where multiple sutures meet and the bones are furthest apart.
What’s normal at the fontanelle
A normal fontanelle: flat or very slightly sunken when the baby is sitting upright and calm; slightly more prominent when lying down (normal — intracranial pressure is slightly higher in this position); pulsating in rhythm with the heartbeat; transiently more full when the baby is crying, straining, or feverish (normal and temporary). It can be touched gently — the membrane is tough and not easily damaged. You can wash, comb, and apply cradle cap treatment over the fontanelle without harm.
Signs that warrant medical assessment
Sunken fontanelle: clearly sunken below the level of the surrounding skull in a baby who is sitting upright — indicates dehydration. Particularly concerning when combined with reduced wet diapers, dry mouth, or lethargy. Seek medical assessment. Bulging fontanelle: fontanelle that protrudes outward when the baby is calm and sitting upright — indicates raised intracranial pressure, which can indicate meningitis, encephalitis, or other serious conditions. This is a medical emergency — call 911 or go to the ER immediately. Early closure (craniosynostosis): a fontanelle closing before 12 months while the skull is still growing may restrict brain expansion — presents as a firm ridge along a suture line rather than a soft groove. Requires specialist assessment.
Practical care
No special care is needed for the fontanelle beyond routine gentle bathing. The misconception that the fontanelle cannot be touched or washed leads to inadequate scalp hygiene and can contribute to cradle cap build-up in the area. Normal shampooing and gentle scalp massage including over the fontanelle area is entirely safe.
Frequently Asked Questions
Why does the fontanelle pulsate?
You are feeling arterial pulse transmitted through the membrane — the rhythmic pressure changes of the arteries beneath. This is completely normal. It may appear more visible when the baby is crying, has a fever, or has a rapid heart rate.
My 4-week-old’s fontanelle looks large — is that normal?
Fontanelle size varies significantly between babies. A large anterior fontanelle at 4 weeks is almost always normal variation. If your doctor or pediatrician has not raised a concern at routine checks, it’s very likely within the normal range.
I can see the fontanelle pulsing and it looks like it’s bulging — should I go to the ER?
Transient bulging when the baby is crying, straining, or lying flat is normal — intracranial pressure transiently increases in these states. A fontanelle that is persistently bulging when the baby is calm, sitting upright, and not crying warrants urgent medical assessment. If uncertain, call 111 or your doctor for guidance.
Related Reading
- Baby’s first check-up: what the pediatrician looks for
- Baby fever: temperature chart, when to go to ER
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