At what age does the posterior fontanelle typically close?

Posterior Fontanelle Closure in Infants

The posterior fontanelle typically closes between 1-3 months of age, much earlier than the anterior fontanelle which closes around 9-13 months of age. 1

Normal Fontanelle Development

Posterior Fontanelle

• Located at the junction of the occipital and parietal bones
• Smaller than the anterior fontanelle at birth
• Closes significantly earlier than the anterior fontanelle
• Typical closure timeline: 1-3 months after birth 1

Anterior Fontanelle (for comparison)

• Average size at birth: approximately 2.1 cm 1
• Median closure time: 13.8 months 1
• In a prospective cohort study of 321 healthy term babies, the mean anterior fontanelle closure time was 9.7 ± 5.0 months (slightly earlier in boys at 9.2 ± 5.1 months compared to girls at 10.3 ± 4.7 months) 2

Clinical Significance of Posterior Fontanelle

While the posterior fontanelle receives less clinical attention than the anterior fontanelle, it serves important functions:

1. Diagnostic window: Can be used as an additional acoustic window for ultrasound examination, particularly for visualization of:

   • Intraventricular hemorrhage 3
   • Periventricular hemorrhage 3
   • Occipital horns of lateral ventricles 4

2. Complementary assessment: Studies have shown that using both anterior and posterior fontanelles for ultrasound examination improves diagnostic accuracy:

   • In one study, intraventricular hemorrhage was detected via the posterior fontanelle but not via the anterior fontanelle on initial diagnostic scans in 14 neonates 4
   • The posterior fontanelle view was particularly valuable for detecting hemorrhage in neonates with normally sized ventricles 4

Warning Signs of Abnormal Posterior Fontanelle

While most clinical guidelines focus on the anterior fontanelle, abnormalities of the posterior fontanelle may include:

• Delayed closure (beyond 3 months)
• Premature closure (before 1 month)
• Unusual size or shape

These findings should prompt consideration of conditions that affect bone development or intracranial pressure.

Clinical Implications

1. Premature closure concerns: Unlike the anterior fontanelle, where premature closure raises concerns about microcephaly, early closure of the posterior fontanelle is generally not associated with microcephaly 2

2. Imaging considerations: The posterior fontanelle serves as an important acoustic window for cranial ultrasound in neonates, particularly for visualizing the occipital horns of the lateral ventricles 4

3. Diagnostic value: When evaluating for intraventricular hemorrhage in premature infants, using both anterior and posterior fontanelles for ultrasound examination significantly improves detection rates compared to using the anterior fontanelle alone 3, 4

Understanding the normal timeline for posterior fontanelle closure helps clinicians distinguish between normal variation and potentially pathological findings that might warrant further investigation.