At what age do the anterior fontanelle and posterior fontanelle typically close?

Timing of Anterior and Posterior Fontanelle Closure

The anterior fontanelle typically closes between 9-18 months of age (average 9.7 months), while the posterior fontanelle closes much earlier, usually by 2-3 months of age. 1

Anterior Fontanelle

Normal Development Pattern

• The anterior fontanelle reaches its maximum size at approximately 1 month of age (average 2.2 cm) 2
• After 6 months of age, the anterior fontanelle size begins to progressively decrease 3
• Mean closure time is 9.7 ± 5.0 months, with slight gender differences (girls: 10.3 ± 4.7 months, boys: 9.2 ± 5.1 months) 1
• By 12 months, approximately 26.5% of anterior fontanelles are closed 2
• By 24 months, approximately 93% of anterior fontanelles are closed 2

Variability Factors

• There is significant normal variation in fontanelle size and closure timing among healthy infants 4, 1
• No significant differences in anterior fontanelle size have been observed between genders, despite boys having larger head circumferences 4
• Racial differences may exist in fontanelle sizes and closure patterns, with studies showing Nigerian infants having larger fontanelles at birth but earlier closure compared to Caucasian and Chinese populations 5
• Fontanelle size is not significantly correlated with weight, length, or head circumference measurements 2

Posterior Fontanelle

• The posterior fontanelle is significantly smaller than the anterior fontanelle at birth
• The posterior fontanelle typically closes much earlier than the anterior fontanelle, usually by 2-3 months of age
• Due to its early closure, the posterior fontanelle has less clinical significance for ongoing monitoring compared to the anterior fontanelle

Clinical Significance

• Ultrasonography through the anterior fontanelle provides an excellent window to examine the infant brain without radiation exposure 6
• Premature closure of the anterior fontanelle may warrant further evaluation including:
  • Head circumference measurement to assess for microcephaly or abnormal head growth 7
  • Neurological examination to assess for signs of increased intracranial pressure 7
  • Consideration of MRI if there are concerns about brain development 7
• Regular developmental surveillance is recommended for infants with premature anterior fontanelle closure 7

Measurement Techniques

• The traditional method measures the anterior-posterior diameter (length) and transverse diameter (width), with the average of these two measurements representing fontanelle size 3
• Oblique measurements have been shown to be equally accurate as the traditional method 1
• Children with fontanelles measuring above the 95th percentile should be monitored closely and investigated further for potentially associated diseases 1