Mechanism of Vertex Delivery

Mechanism of Vertex Delivery

1. Overview of the Delivery Mechanism:

The delivery mechanism is the process by which the fetus moves from the uterus to the outside world through the vagina. Regardless of the fetal presentation, the delivery mechanism goes through three main stages: delivery of the head, delivery of the shoulders, and delivery of the buttocks. Among these, delivery of the head is the most challenging stage, as the head is the largest and hardest part of the fetus, making it difficult to reduce its diameter compared to the shoulders and buttocks.

The fetus is most affected by the pelvic bones during its journey from the uterus to the outside world.

Each stage of delivery is divided into four phases: engagement, descent, rotation, and expulsion. The phases of engagement and descent occur simultaneously, as do the phases of rotation and expulsion.

• Engagement: The diameter of the presenting part passes through the plane of the pelvic inlet.

• Descent: The largest diameter of the presenting part moves from the plane of the pelvic inlet to the plane of the pelvic outlet, meaning from the plane of engagement to the plane of expulsion.

• Rotation: The landmark of the presenting part rotates forward, lying under the symphysis pubis or rotating forward towards the sacrum. Generally, forward presentations rotate less (45 degrees), while posterior presentations rotate more (135 degrees). Therefore, the rotation time in posterior presentations is usually longer than in anterior presentations.

• Expulsion: The presenting part exits the plane of the pelvic outlet, or the plane of expulsion. Clinically, expulsion occurs when the presenting part emerges from the vulva.

2. Mechanism of Vertex Delivery in the Left Occiput Anterior Presentation:

2.1. Delivery of the Head:

• Engagement phase:

• The fetal head is not yet fully engaged, with the occipitofrontal diameter (11.5 cm) parallel to the plane of the pelvic inlet.

• Preparation for engagement: The fetal head flexes, transitioning to the suboccipitobregmatic diameter (9.5 cm) parallel to the left oblique diameter. At this point, the head touches the chest.

• Full engagement: The suboccipitobregmatic diameter coincides with the plane of the pelvic inlet along the left oblique diameter.

• Descent phase:

• As the head descends, it reaches the pelvic floor, causing the pelvic floor to thin out. Engagement and descent often occur simultaneously.

• The fetal position also changes: the fetal body and spine straighten. When the presenting part descends fully past the chest and neck, the fetus will extend and its spine will curve forward.

• Rotation phase:

• The head rotates 45 degrees (internal rotation), at which point the shoulders are parallel to the left oblique diameter.

• Rotation begins when the fetal head touches the pelvic floor.

• The pressure from uterine contractions pushes the fetus downward, while the pelvic floor in front acts as a barrier, causing the fetal head to rotate along the anteroposterior diameter.

• The second factor is the shape of the head, which causes the presenting part to rotate.

• Once rotation is complete, the fetal head occupies the entire lower portion of the birth canal.

• The occiput of the fetus lies under the symphysis pubis.

• Expulsion phase:

• The fetal head remains flexed, the fetal body extends as much as possible, and the spine curves forward.

• Preparation for expulsion: The fetal head continues to flex due to the pressure from uterine contractions, abdominal muscle contractions, and resistance from the pelvic floor. The head flexes to allow a portion of the parietal bone to escape the plane of the pelvic outlet. The lower edge of the occipital bone scrapes against the symphysis pubis, and the head stops flexing.

• Full expulsion: The fetal head extends gradually, the pelvic floor is compressed by the brow-face, causing the perineum to bulge and elongate. The fetal head rests against the lower edge of the symphysis pubis. Under the pressure of uterine contractions, the head gradually extends to allow the suboccipitomental diameter to exit the vulva.

• The chin emerges from the vulva, ending the head delivery stage.

• After expulsion, the fetal head rotates 45 degrees to return to its previous position (to the level of +4, external rotation 45 degrees, back to its original position).

2.2. Delivery of the Shoulders:

• Engagement phase:

• The bisacromial diameter is perpendicular to the suboccipitobregmatic diameter. The shoulders engage along the right oblique diameter.

• Preparation for engagement: The shoulders reduce in size from 12 cm to 9.5 cm.

• Full engagement: The bisacromial diameter passes through the plane of the pelvic inlet along the right oblique diameter. Uterine contractions push the shoulders downward to pass through the plane of engagement.

• Descent phase:

• The shoulders descend from the plane of the pelvic inlet to the plane of the pelvic outlet.

• Rotation phase:

• The shoulders rotate 45 degrees, the bisacromial diameter coinciding with the anteroposterior diameter of the pelvic outlet.

• Expulsion phase:

• Under the pressure of uterine contractions and the mother’s pushing effort, the anterior shoulder is expelled. The anterior shoulder expels to the lower edge of the deltoid muscle (the birth attendant pulls the fetus downward).

• The posterior shoulder is expelled, pulling the fetus upward.

• At this point, both shoulders have emerged.

2.3. Delivery of the Buttocks:

• The mechanism of delivery for the buttocks is similar to that of the shoulders, as the largest diameter of the buttocks is the intertrochanteric diameter. This diameter is parallel to the bisacromial diameter, so the phases of engagement, descent, rotation, and expulsion are all the same.

• Clinically, delivery of the buttocks happens very quickly after delivery of the shoulders and is easily accomplished.

3. Caring for the Mother During the Pushing Stage:

It is important to have a firm grasp of the delivery mechanism to know when to intervene during the pushing stage.

3.1. Assessment:

• The mother’s health status.

• Fetal presentation, position, and level of engagement.

• Which phase of the pushing stage is taking place when assisting the mother in pushing.

3.2. Nursing Diagnosis:

• Pain from uterine contractions.

• Dry mouth due to mouth breathing.

• Risk of perineal laceration due to rapid delivery.

3.3. Care Plan:

• Provide emotional support.

• Assist the mother during delivery of the occiput, delivery of the shoulders, and delivery of the buttocks.

• Administer oxytocin medication if prescribed.

3.4. Implementing the Care Plan:

• Be empathetic and encouraging to the mother, avoid yelling or causing unnecessary distress.

• Allow the mother to drink small amounts of water between uterine contractions to quickly regain strength.

• Provide cooling measures.

• Monitor the mother’s vital signs.

• When uterine contractions occur: assist in properly flexing the fetal head by:

• Gently pressing on the vertex with one hand while holding the perineum firmly with the other.

• When the occiput of the fetal head is under the symphysis pubis:

• Assist in gradually extending the fetal head with each uterine contraction and the mother’s pushing effort.

• When the brow-face diameter (10.5 cm) appears, which increases the risk of perineal tearing:

• Help the head expel using an asymmetrical method to protect the perineum: one parietal bone emerges first, followed by the other.

• When the shoulders begin to engage and the head is fully delivered, the shoulders have descended and rotated:

• Direct the fetal head downward toward the floor to allow the anterior shoulder to slowly exit to the lower edge of the deltoid muscle. Once the anterior shoulder is expelled, focus on delivering the posterior shoulder.

• The posterior shoulder is located below the sacrum, so it is lower than the perineum. Therefore, expulsion can easily cause perineal tears.

• Use one hand to guide the fetal head upward toward the ceiling to deliver the posterior shoulder while the other hand protects the perineum to prevent excessive bulging, which can cause perineal tearing.

3.5. Evaluation:

• Nurses should have a solid understanding of the delivery mechanism to ensure a complete and safe delivery.

• After delivery, the newborn should be stable, the mother should be well, her hemodynamics should be stable, the lochia should be normal, and the outcome should be favorable. Report any abnormal symptoms, such as heavy bleeding, poor uterine contraction, or any other unusual signs, to the physician immediately.

4. Multiple-Choice Questions:

• 1. Vertex presentation is:

• A. The head is well flexed.

• B. The head is maximally extended.

• C. The head is in neither a flexed nor extended position.

• D. The head is slightly flexed.

• E. The landmark is the anterior fontanel.

• Answer: A

• Content:

• A. Engagement is when the diameter of the presenting part passes through the plane of the pelvic inlet. Correct.

• B. Expulsion is when the presenting part exits the plane of the pelvic outlet, or the plane of expulsion. Correct.

• C. For all fetal presentations, the delivery mechanism goes through three stages: delivery of the head, delivery of the shoulders, and delivery of the feet. Incorrect. Delivery of the feet is a unique fetal presentation, with a different delivery mechanism than other presentations.

• D. During the shoulder delivery phase, the posterior shoulder usually emerges first. Incorrect. The anterior shoulder usually emerges first, followed by the posterior shoulder.