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Posterior Fontanelle

fonticulus posterior

The posterior fontanelle is the small triangular soft spot at the back of a newborn's head, where the two parietal bones meet the occipital bone. It lies at the future site of lambda, at the junction of the sagittal and lambdoid sutures. It is much smaller than the anterior fontanelle and closes within the first few months of life.

Anatomical Data

Structure & Landmarks

Articulations	Right and left parietal bones Occipital bone
Landmarks	Triangular gap at the junction of the sagittal and lambdoid sutures Future site of lambda
Ossification	A membrane-covered gap between the parietal bones and the occipital squama that usually closes by about 2 to 3 months after birth.

Clinical Relevance

Clinical Notes

During labour, the posterior fontanelle is felt on vaginal examination to work out the position of the baby's head; in the usual occipito-anterior position it lies toward the front of the mother's pelvis. A posterior fontanelle that is still open and large after the first months may indicate congenital hypothyroidism or raised intracranial pressure. It is also used as an acoustic window for ultrasound of the back of the infant brain.

Pathology

Common Injuries & Conditions

Large Posterior Fontanelle

An unusually large or persistently open posterior fontanelle, which can be a sign of congenital hypothyroidism, skeletal dysplasia or raised intracranial pressure and prompts further testing.

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