When Does the Anterior Fontanelle Close in Babies?
The anterior fontanelle typically closes between 9-18 months of age, with a median closure time of 13.8 months. 1
Normal Fontanelle Development Timeline
- At birth: Average size is approximately 2.1 cm 1
- 1 month: Fontanelle reaches its maximum size of about 2.2 cm 2
- 6 months: Only about 3% of fontanelles are closed 2
- 12 months: Approximately 26.5% of fontanelles are closed, with average size reduced to 1.0 cm 2
- 24 months: About 93% of fontanelles are closed 2
Factors Affecting Closure Time
- Gender differences: Boys tend to have slightly earlier closure than girls (9.2 ± 5.1 months for boys vs. 10.3 ± 4.7 months for girls) 3
- Normal variation: The range of normal closure can be quite wide, from 4 to 26 months 4
- No correlation: Fontanelle size is not significantly correlated with weight, length, or head circumference measurements 2
Clinical Significance
- A persistent open fontanelle beyond 24 months warrants further investigation but can sometimes be a normal variant 4
- Abnormally early closure may suggest craniosynostosis
- Delayed closure beyond expected timeframes may indicate:
- Hypothyroidism
- Down syndrome
- Achondroplasia
- Increased intracranial pressure
- Rickets 1
Assessment Methods
- Traditional method: Measuring the anterior-posterior and transverse diameters
- Oblique measurements: Equally accurate and can be used interchangeably with the traditional method 3
- Fontanelle assessment should include evaluation of:
- Size
- Tension (normal, bulging, or sunken)
- Pulsations 5
Warning Signs Requiring Urgent Evaluation
- Bulging fontanelle: May indicate increased intracranial pressure from meningitis, hemorrhage, hydrocephalus, or tumors 5
- Sunken fontanelle: Usually indicates dehydration 5
- Rapidly increasing head circumference with abnormal fontanelle tension 5
Key Points for Clinicians
- The anterior fontanelle serves as an important "window to the brain" for clinical assessment and ultrasonography in infants
- Children with fontanelles measuring above the 95th percentile should be monitored closely and investigated further 3
- A persistent open fontanelle beyond 24 months in an otherwise healthy child with normal development may be a normal variant, but should be evaluated to rule out underlying conditions 4