In a normally developed term infant, which fontanelles (anterior and/or posterior) are normally open at birth?

Fontanelles in Normally Developed Term Infants

In a normally developed term infant, both the anterior and posterior fontanelles are normally open at birth. 1, 2, 3

Normal Fontanelle Characteristics

• The anterior fontanelle is located between the frontal and parietal bones and serves as an important anatomical diagnostic tool in the assessment of skull and brain development 4
• The posterior fontanelle is located between the occipital and parietal bones 1
• The anterior fontanelle is typically larger than the posterior fontanelle at birth 3
• In full-term infants, the anterior fontanelle width (transversal diameter) at birth averages about 2.0 cm 2
• The size of fontanelles can vary significantly between individuals while still being considered normal 2, 3

Fontanelle Closure Timeline

• The mean anterior fontanelle closure time is approximately 9.7 ± 5.0 months (slightly earlier in boys at 9.2 ± 5.1 months compared to girls at 10.3 ± 4.7 months) 3
• The posterior fontanelle typically closes earlier than the anterior fontanelle, usually within the first 2-3 months of life 4
• By 12 months of age, the anterior fontanelle width typically decreases to about 0.6 cm in most infants 2

Clinical Significance of Fontanelles

• Fontanelles allow for brain growth without compression by the skull during infancy 4
• They serve as acoustic windows for cranial ultrasonography, particularly important in neonatal assessment 1, 5
• The anterior fontanelle can be used to assess intracranial pressure through palpation (fullness or bulging may indicate increased pressure) 1
• Premature closure of fontanelles may indicate craniosynostosis and requires further evaluation 6
• Abnormally large fontanelles may be associated with various conditions including hydrocephalus, hypothyroidism, or genetic syndromes 7

Diagnostic Considerations

• Cranial ultrasonography through the anterior fontanelle is a standard technique for evaluating brain structures in infants 1, 5
• The posterolateral fontanelle approach can provide better visualization of posterior fossa abnormalities that may not be visible through the anterior fontanelle 5
• Monitoring fontanelle size and closure timing can help identify potential abnormalities in development 3
• Measurement of head circumference and plotting it on growth charts should accompany fontanelle assessment 6

Common Pitfalls and Caveats

• Mistaking a normal variation in fontanelle size for pathology; there is significant normal variation in fontanelle size among healthy infants 2, 3
• Failing to recognize that early fontanelle closure is not always associated with microcephaly 3
• Overlooking the posterior fontanelle during newborn assessment, which is smaller but should still be palpable at birth 1
• Relying solely on anterior fontanelle examination when assessing for posterior fossa abnormalities; the posterolateral approach provides better visualization of these structures 5