Where are the skull fontanelles located?
These anatomical structures are situated at the intersection points of the cranial sutures in the fetus and newborn. The lateral fontanelles are closed by the time of birth. The two primary fontanelles of greatest practical importance are:
- Anterior fontanelle (fonticulus major / anterior) — a diamond-shaped structure at the junction of the parietal and frontal bones, where the sagittal, frontal (metopic), and coronal sutures meet.
- Posterior fontanelle (fonticulus minor / posterior) — a triangular structure at the junction of the parietal and occipital bones, where the sagittal and lambdoid sutures meet.
What are their structure and dimensions?
The cranial bones of a newborn have a moderate density. In a term infant, the size of the anterior fontanelle is approximately 1.5–2.5 × 2–3 cm. The posterior fontanelle is a small depression that in a mature fetus is typically covered by a fibrous plate or filled with bone.
At birth, the skeletal system contains a large volume of cartilage tissue.
What is the obstetric significance of the fontanelles?
Palpatory differentiation of the anterior and posterior fontanelles by the number of converging sutures is critical for determining fetal head engagement during vaginal examination in labor:
- Four sutures converge at the anterior fontanelle.
- Three sutures converge at the posterior fontanelle, where the sagittal suture terminates.
The position of the sagittal suture, along with the anterior and posterior fontanelles, helps determine the fetal position and presentation, the type of engagement (synclitism or asynclitism), and the current stage of the labor mechanism (flexion or extension).
Clinical significance and closure timing
The presence of sutures and fontanelles allows for cranial molding, which is necessary to overcome spatial constraints within the birth canal.
- Posterior fontanelle: closed in most infants at birth, but remains open in about 25% of newborns and typically closes by the 8th week of life.
- Anterior fontanelle: open at birth; its physiological closure occurs between 12 and 18 months of age.
- Age-related considerations: in newborns and infants with hypertensive hydrocephalus, progressive head enlargement is a key clinical sign because the fontanelles and sutures are not yet fully fused.