Where are the skull fontanelles located?
The anatomical structures are located at the junction points of the cranial sutures of the fetus and newborn. The lateral fontanelles are closed by the time of birth. The two main fontanelles are of the greatest practical importance:
- Anterior fontanelle (fonticulus major) — a diamond-shaped structure at the junction of the parietal and frontal bones, at the intersection of the sagittal, frontal, and coronal sutures.
- Posterior fontanelle (fonticulus minor) — a triangular structure at the junction of the parietal and occipital bones, at the intersection of the sagittal and lambdoid sutures.
What is the structure and dimensions of the structures?
The bones of the newborn's skull have moderate density. In a full-term infant, the size of the anterior fontanelle is 1.5–2.5 × 2–3 cm. The posterior fontanelle is a small depression which in a mature fetus is closed by a fibrous plate or filled with bone.
By the time of birth, the skeletal system contains a large amount of cartilage tissue.
What is the obstetrical significance of the fontanelles?
Palpatory differentiation of the anterior and posterior fontanelles by the number of converging sutures is critically important for determining fetal head engagement during vaginal examination in labor:
- 4 sutures converge in the anterior fontanelle.
- 3 sutures converge in the posterior fontanelle, and the sagittal suture terminates in it.
Based on the location of the sagittal suture, the anterior and posterior fontanelles, the fetal position, presentation, type of engagement (synclitic or asynclitic), and the current stage of the mechanism of labor (flexion or extension) are determined.
Clinical significance and closure timeframes
The presence of sutures and fontanelles ensures the molding of the head necessary to overcome spatial obstacles in the birth canal.
- The posterior (occipital) fontanelle is closed at birth in the majority, but remains open in 25% of newborns and closes by the 8th week.
- The anterior fontanelle is open at birth, and its physiological closure occurs between 12 and 18 months.
- Age-related features: in newborns and young children with hypertensive hydrocephalus, the leading symptom is an increase in head size, as the fontanelles and sutures are not yet fully closed.
The anterior fontanelle is a rhombus with 4 sutures; the posterior is a triangle with 3 sutures.
Class and exam questions
Where is the anterior fontanelle localized and what is its shape?
The anterior fontanelle has a diamond shape and is located at the intersection of the sagittal, frontal, and coronal sutures, between the parietal and frontal bones.
How many sutures converge in the posterior fontanelle?
3 sutures converge in the posterior fontanelle; the sagittal suture terminates in it.
What are the timeframes for the physiological closure of the anterior fontanelle?
Physiological closure of the anterior fontanelle occurs between 12 and 18 months.
What is the obstetrical significance of the skull fontanelles?
The sutures and fontanelles are used to determine fetal head engagement, position, presentation, and stage of the mechanism of labor, while the presence of sutures and fontanelles ensures head molding during passage through the birth canal.
What else to study on this topic
- Cranial sutures (sutures cranii)
- Fetal head dimensions
- Biomechanism of labor in occiput presentation
- Planes of the lesser pelvis
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