Fetal Skull Anatomy

• Fetal skull characteristics, including incomplete ossification, fontanelles, sutures, and proportions, facilitate molding during childbirth.

Bones of the Fetal Skull

• Frontal, parietal, occipital, and temporal bones constitute the fetal skull.
• These bones are not fused, which permits movement.
• The frontal bone consists of two halves divided by the frontal suture, typically fusing by age 8.
• Parietal bones are the fetal skull's largest.
• The occipital bone forms the skull's posterior and base.
• Temporal bones are located on the skull's sides and contain inner ear structures.

Sutures

• Sutures are fibrous joints connecting the bones.
• Sagittal, coronal, lambdoid, and metopic sutures are the primary sutures.
• The sagittal suture is located between the parietal bones.
• The coronal suture is located between the frontal and parietal bones.
• The lambdoid suture is located between the occipital and parietal bones.
• The metopic suture is located between the two halves of the frontal bone (if unfused).
• Sutures facilitate skull deformation during birth, reducing the head's diameter.
• Sutures gradually ossify after birth, eventually fusing.
• Premature suture fusion (craniosynostosis) can impede brain growth and change skull shape.

Fontanelles

• Fontanelles are membrane-covered spaces located where several sutures meet.
• Anterior and posterior fontanelles are major ones.
• The anterior fontanelle, located at the junction of the sagittal, coronal, and frontal sutures, is diamond-shaped and closes between 9-18 months.
• The posterior fontanelle, located at the junction of the sagittal and lambdoid sutures, is triangular and closes by 2-3 months.
• Fontanelles allow for skull molding and brain growth.
• They also allow assessment of hydration and intracranial pressure in infants.
• Sunken fontanelles may indicate dehydration, while bulging may indicate increased intracranial pressure.

Diameters of the Fetal Skull

• Key fetal skull diameters are important for vaginal delivery.
• The biparietal diameter (BPD), the distance between parietal eminences, is about 9.5 cm at term.
• The occipitofrontal diameter, from the occipital protuberance to the frontal bone, is about 11.5 cm.
• The suboccipito-bregmatic diameter, from the inferior occiput to the bregma (anterior fontanelle), is about 9.5 cm when the head is well-flexed.
• The mento-vertical diameter, from the chin to the highest point on the sagittal suture, measures about 13.5 cm when the head is deflexed.
• Engagement of the fetal head into the pelvis usually occurs with the smallest diameter presenting.

Molding

• Molding is the change in the shape of the fetal skull during passage through the birth canal.
• The skull bones overlap at the sutures, reducing the diameter.
• The degree of molding varies with labor duration and difficulty.
• Excessive molding can cause cephalohematoma or caput succedaneum.
• Caput succedaneum is swelling of the scalp due to pressure during labor.
• Cephalohematoma is a collection of blood between the skull bone and its periosteum.

Facial Bones

• The fetal face is small compared to the cranium.
• Facial bones include the maxilla, mandible, nasal bones, and zygomatic bones.
• The orbits (eye sockets) are relatively large.
• The mandible (jawbone) is small but well-developed.

Clinical Significance

• Fetal skull anatomy knowledge is crucial for obstetricians and healthcare providers.
• Assessing fetal head position and size during labor is essential for managing delivery.
• Ultrasound measures BPD and other skull diameters.
• Abnormal fetal head size or position can lead to obstructed labor, requiring intervention.
• Suspected craniosynostosis requires evaluation and potential surgical correction.
• Understanding fontanelles is important for assessing infant health and development.

Ossification

• Ossification begins during embryonic development and continues after birth.
• Intramembranous ossification forms the flat bones - frontal and parietal bones.
• Endochondral ossification forms the base of the skull.
• Ossification centers appear within the bones, gradually expanding and fusing.
• Fontanelles and sutures allow for continued brain growth despite ossification.
• Vitamin D is crucial for proper bone mineralization.

Skull Base

• The skull base is formed by the occipital, temporal, and sphenoid bones.
• The foramen magnum, through which the spinal cord passes, is in the occipital bone.
• Temporal bones house the inner ear structures responsible for hearing and balance.
• The sphenoid bone is a complex bone that forms part of the base of the skull and the orbits.
• The skull base provides support and protection for the brainstem and other vital structures.

Proportions

• The fetal skull has a larger cranial vault relative to the face, compared to adults.
• The forehead is prominent, and the facial features are less defined.
• The orbits are large, and the nasal bridge is relatively flat.
• As the child grows, the facial bones develop, and the proportions shift.
• By adulthood, the face makes up a larger proportion of the overall skull.

The fetal skull has unique features like fontanelles and unfused sutures that enable molding during birth. It comprises frontal, parietal, occipital, and temporal bones, connected by sutures such as sagittal, coronal, lambdoid, and metopic.