Fetal Skull – Full Obstetrics Notes with Diagrams and Clinical Tips

1. Introduction

• The fetal skull is the most important part of the fetus during labor and delivery.
• It is large, bony, and firm, and plays a key role in labor progress and delivery outcome.

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2. Divisions of Fetal Skull

• The fetal skull is divided into:
  • Vault of the skull (calvaria) – dome-shaped, formed by flat bones.
  • Base of the skull – more ossified and less compressible.
  • Face – not important in obstetrics.

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3. Bones of the Vault

• The vault includes:
  • 2 frontal bones
  • 2 parietal bones
  • 1 occipital bone
• These bones are connected by membranous sutures and fontanelles, allowing molding during labor.

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4. Sutures of the Fetal Skull

• Definition: Narrow membranous joints between the bones of the skull.
• Important Sutures:
• Sagittal Suture – between two parietal bones.
• Coronal Sutures – between frontal and parietal bones.
• Lambdoid Sutures – between parietal and occipital bones.
• Frontal/Metopic Suture – between the two frontal bones.

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5. Fontanelles

• Definition: Membranous gaps at junctions of sutures.
• Important Fontanelles:
  • Anterior Fontanelle (Bregma):
    • Diamond-shaped.
    • Located at junction of sagittal, coronal & frontal sutures.
    • Closes by 18 months.
  • Posterior Fontanelle (Lambda):
    • Triangular.
    • Located at junction of sagittal and lambdoid sutures.
    • Closes by 6–8 weeks.

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6. Diameters of the Fetal Skull

• Important in labor for determining engagement and passage through the birth canal.
• Anteroposterior Diameters:
  • Suboccipitobregmatic – 9.5 cm (well-flexed head).
  • Suboccipitofrontal – 10 cm.
  • Occipitofrontal – 11.5 cm.
  • Mentovertical – 13.5 cm.
  • Submentobregmatic – 9.5 cm (face presentation).
  • Submentovertical – 11.5 cm.
• Transverse Diameters:
  • Biparietal – 9.5 cm (widest transverse diameter).
  • Bitemporal – 8.0 cm (narrowest fixed transverse diameter).

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7. Molding of the Fetal Skull

• Definition: Overlapping of skull bones during labor to reduce head size.
• Mechanism: Parietal bones override frontal and occipital bones.
• Clinical Significance:
• Helps fetal head pass through pelvis.
• Excessive molding may indicate obstructed labor.

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8. Caput Succedaneum

• Definition: Edema of fetal scalp due to pressure against cervix.
• Location: Over presenting part, crosses suture lines.
• Disappears: Within 24–48 hours after birth.

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9. Cephalhematoma

• Definition: Collection of blood beneath the periosteum of skull bone.
• Location: Confined to one bone, does not cross sutures.
• Disappears: Gradually over weeks.

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10. Engagement of Fetal Head

• Definition: When the biparietal diameter passes through the pelvic brim.
• Indicates: Beginning of labor progression.
• Can be confirmed by clinical or ultrasound examination.

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11. Clinical Importance of Fetal Skull

• Guides management during:
  • Labor progression
  • Position of fetal head
  • Instrumental delivery (forceps/vacuum)
• Helps identify malpresentations and decide delivery route.

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12. Summary Points

• Fetal skull is compressible due to sutures and fontanelles.
• Key diameters influence labor outcomes.
• Molding helps the fetal head adapt during birth.
• Caput succedaneum and cephalhematoma are common post-labor findings.

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Tips for Students

• Remember the BPD (biparietal diameter) = 9.5 cm – most important for engagement.
• Sutures & fontanelles help identify position during vaginal examination.
• Know the difference between caput (soft, crosses sutures) and cephalhematoma (firm, does not cross sutures).

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