At what ages do the anterior and posterior fontanelles normally close, and how does prematurity affect their timing?

Fontanelle Closure Timing

The anterior fontanelle typically closes between 9-14 months of age (median 13.8 months), while the posterior fontanelle closes much earlier, usually by 2-3 months of age. 1, 2

Anterior Fontanelle Closure

Normal Timing in Term Infants

• Median closure occurs at 13.8 months of age, though there is substantial normal variation 1
• Mean closure time is approximately 9.7 ± 5.0 months 2
• Closure frequency increases steadily: 16% closed by 10 months, 53% by 16 months, and 88% by 20 months 3
• Early closure (5-6 months) occurs in 3-5% of healthy infants and may represent a normal variant 3
• The fontanelle can remain open up to 24 months in some normal children 4, 2

Size Variation

• Average size at birth is 2.1 cm (transverse diameter approximately 2.0 cm at the 50th percentile) 1, 5
• Size increases slightly in the first 2 months, then decreases progressively 5, 3
• By 12 months, the width decreases to approximately 0.6 cm 5
• Great variability exists in both fontanelle size and closure timing among healthy infants 4, 2

Effect of Prematurity

Preterm Infant Differences

• In appropriate-for-gestational-age (AGA) preterm infants, the anterior fontanelle increases from 1.8 cm at 32 weeks postmenstrual age to 2.5 cm at 40 weeks—significantly larger than term infants at birth 5
• By 3 months corrected age, there is no significant difference in fontanelle size between preterm and term infants 5
• Age at closure shows no significant difference between term and preterm infants when measured from birth 4

Sex Differences

• Girls tend to have slightly later closure (10.3 ± 4.7 months) compared to boys (9.2 ± 5.1 months) 2
• Boys may have slightly smaller fontanelles despite larger head circumferences, though differences are not statistically significant 5

Clinical Implications

When to Investigate Further

• Children with fontanelles measuring above the 95th percentile should be monitored closely and investigated for associated diseases (hypothyroidism, Down syndrome, achondroplasia, rickets, increased intracranial pressure) 1, 2
• Premature closure requires immediate head circumference measurement and neurological examination to assess for craniosynostosis or microcephaly 6
• Early fontanelle closure is not necessarily associated with microcephaly and may be a normal variant 2

Assessment Approach

• Both traditional (anterior-posterior and transverse) and oblique measurements are equally accurate 2
• Ultrasound examination through the fontanelle is the preferred imaging modality when needed, avoiding radiation exposure 7
• CT imaging should be reserved only for surgical planning in selected cases, not routine diagnosis 6