Makindo Medical Notes

Overview

The position and size of the fetal head play a significant role in the labor and delivery process. Understanding fetal head anatomy, diameters, and the mechanics of vaginal birth is crucial for safely managing labor and recognizing when interventions may be needed.

Bones of the Fetal Skull

• The fetal skull is made up of several bones separated by sutures, allowing for flexibility during birth.
• Main bones:
  • Frontal bones: Form the forehead region.
  • Parietal bones: Located on each side of the skull.
  • Occipital bone: Forms the back of the head.
  • Temporal bones: Situated at the sides of the head.

Sutures and Fontanelles

• Sutures: Fibrous joints between the bones of the skull allowing for overlapping (molding) during birth.
  • Coronal suture: Between the frontal and parietal bones.
  • Sagittal suture: Between the parietal bones.
  • Lambdoid suture: Between the parietal and occipital bones.
• Fontanelles: Soft spots where sutures intersect, allowing for skull flexibility.
  • Anterior fontanelle: Diamond-shaped, located at the intersection of the coronal and sagittal sutures.
  • Posterior fontanelle: Triangular, at the intersection of the sagittal and lambdoid sutures.

Fetal Head Diameters Important for Vaginal Birth

• Suboccipitobregmatic (9.5 cm): The smallest diameter, ideal for passage through the birth canal.
• Occipitofrontal (11.0 cm): Diameter from the occiput to the frontal bone; common in flexed head presentations.
• Mentofrontal (13.5 cm): Diameter from chin (mentum) to frontal bone; encountered in brow presentations.
• Submentobregmatic (9.5 cm): Seen in face presentations.

Mechanisms of Labor

The movement of the fetal head through the birth canal is facilitated by a series of cardinal movements, allowing the smallest diameters of the fetal head to match the maternal pelvis at each stage. These steps are:

• Engagement: The fetal head enters the pelvic inlet, usually in an occiput-transverse position, where the biparietal diameter aligns with the pelvic brim.
• Descent: The fetal head moves down into the pelvis due to uterine contractions, maternal pushing, and gravity.
• Flexion: The fetal chin moves toward the chest, allowing the smallest diameter (suboccipitobregmatic) to present first.
• Internal Rotation: The head rotates to align the occiput anteriorly, fitting the longest fetal head diameter to the widest maternal pelvis diameter.
• Extension: The fetal head extends as it passes under the pubic symphysis, allowing the occiput, brow, and then face to emerge.
• External Rotation (Restitution): Following delivery of the head, the fetal head rotates to align with the shoulders, which are now descending through the pelvis.
• Expulsion: Finally, the shoulders and rest of the body are delivered, completing the birth.

Common Fetal Head Positions During Labor

• Occiput Anterior (OA): This is the optimal position, where the fetal occiput faces the mother's front. It allows for the easiest passage through the birth canal.
• Occiput Posterior (OP): In this position, the fetal occiput faces the mother's back. It often causes prolonged labor and increases the chance of operative delivery.
• Transverse Position: The fetal head is sideways, with the occiput facing one of the mother's hips. This position may require manual rotation or assisted delivery.

Complications Related to Fetal Head Position

• Cephalopelvic Disproportion (CPD): Occurs when the fetal head is too large or positioned unfavorably for the maternal pelvis.
• Face or Brow Presentation: Presenting diameters are larger, which can obstruct labor and may require cesarean delivery.
• Shoulder Dystocia: When the shoulders fail to deliver following the head; requires immediate interventions to prevent complications.

Key Points for Clinical Practice

• Assessing fetal position and head engagement is crucial for labor management.
• Understanding the cardinal movements can aid in identifying issues during delivery.
• Prompt intervention in cases of malposition or disproportion can reduce maternal and fetal complications.

Understanding fetal head anatomy, positions, and movements during birth is critical for managing labor effectively and ensuring a safe delivery for both mother and baby.