Mechanism of Labor Notes

Normal Labor and Fetal Positions

Normal labor is characterized by a longitudinal lie of the fetus, cephalic presentation, vertex as the presenting part, occiput as the denominator, with the most common position being LOT (Left OccipitoTransverse). In a cephalic presentation, the fetal head is the presenting part, making vaginal delivery feasible. The vertex presentation is the most common and safest, where the occiput (back of the fetal head) is the reference point. Normal vaginal delivery can occur from positions one to five, but the explanation will focus on the LOT position, which is the most frequently encountered.

Cardinal Movements

The movements a fetus makes while traversing the maternal pelvic canal are known as cardinal movements, totaling seven. These movements are crucial for the fetus to navigate the bony pelvis efficiently, adapting to its changing diameters and contours.

Fetal Skull Anatomy

  • Anterior Fontanelle: Diamond-shaped and located at the junction of the frontal and parietal bones. It allows for molding of the fetal head during passage through the birth canal and typically closes between 9 to 18 months after birth.

  • Posterior Fontanelle: Triangular, also known as lambda, and lies closer to the occipital bone. Its position helps determine the fetal position during labor. It is at the junction of the sagittal and lambdoid sutures, typically closes 6 to 8 weeks after birth.

  • Frontal Bones: Left and right, separated by the frontal suture, which usually fuses by age 2.

  • Parietal Bones: Left and right, separated by the sagittal suture, which runs along the midline of the skull.

  • Coronal Suture: Separates the frontal and parietal bones, running from one side of the skull to the other.

  • Lambdoid Suture: Separates the parietal and occipital bones, shaped like the Greek letter lambda (Λ).

Pelvic Views and Mirror Images

Pelvic images are typically shown from below, presenting a mirror image. This means what appears on the left in the image is the mother's right side, and vice versa. Clinicians must mentally adjust for this mirror effect when interpreting images to accurately determine fetal position.

Identifying Fetal Positions

  • LOT (Left OccipitoTransverse): Occiput faces the transverse direction on the left side of the mother's pelvis.

To identify the LOT position, the sagittal suture is seen lying in the transverse diameter of the pelvis. The posterior fontanelle is towards the right side in the image (mother's left side), indicating the occiput is on the left side of the mother.

Engagement

Engagement occurs when the largest transverse diameter of the fetal head (biparietal diameter), measuring approximately 9.5 cm, crosses the pelvic inlet or pelvic brim. This signifies that the fetal head has entered the true pelvis.

  • Timing: In primigravida females (first pregnancy), engagement typically occurs at 38 weeks of pregnancy, often before the onset of labor. In multigravida females (previous pregnancies), it occurs at the onset of labor, as the pelvic muscles and tissues are more relaxed and accommodating.

Clinical Assessment of Engagement

Engagement can be assessed via per abdominal or per vaginal examination, each providing distinct information about the fetal head's position.

Per Abdominal Examination

The fetal head is arbitrarily divided into five equal parts to estimate the proportion of the head palpable above the pubic symphysis. Engagement is determined by the number of parts palpable above this landmark.

  • Five-fifths (5/5) palpable: The entire head is felt, indicating the head has not entered the pelvis (free floating).

  • Two-fifths (2/5) or less palpable: Engagement has occurred, meaning the majority of the fetal head has passed through the pelvic inlet.

Leopold's Maneuvers

Leopold's maneuvers are a series of four specific steps used to palpate the fetal position through the maternal abdomen.

  • Leopold's Third Maneuver: Assesses the mobility of the head to determine if it is engaged.

    • If the head is free floating (5/5 palpable), it is easily movable and ballottable, meaning it can be pushed gently from side to side.

  • Leopold's Fourth Maneuver: Determines engagement by assessing if the fingers converge or diverge below the presenting part.

    • If the head is free floating (5/5 palpable), the fingers converge because they can easily meet below the unengaged head.

If the head has entered the pelvis but is not yet engaged (e.g., 4/5 or 3/5 palpable), it is considered fixed but not engaged. In this case, Leopold's third maneuver will show the head is not ballottable, and Leopold's fourth maneuver will show the fingers diverge, as the engaged portion of the head obstructs their convergence.

Per Vaginal Examination

This involves assessing the station of the fetal head, which is its position relative to the ischial spines of the maternal pelvis.

Station of Fetal Head

Station refers to the position of the fetal head relative to the ischial spines. The ischial spines serve as a key landmark in assessing descent.

  • Zero Station: The leading point of the fetal head is at the level of the ischial spines, indicating engagement.

  • Negative Station: The fetal head is above the ischial spines (e.g., -1, -2, -3 cm), with negative numbers indicating the distance in centimeters above the spines.

  • Positive Station: The fetal head is below the ischial spines (e.g., +1, +2, +3 cm), with positive numbers indicating the distance in centimeters below the spines.

Engagement per vaginally is said to occur when the station is at zero or below, confirming that the biparietal diameter has passed through the pelvic inlet.

Clinical Importance of Engagement

Engagement indicates that the largest transverse diameter has crossed the pelvic inlet, suggesting no cephalopelvic disproportion (CPD) at the level of the inlet. Cephalopelvic disproportion means the fetal head is too large or the maternal pelvis is too small to allow the fetus to pass through.

Synclitic and Asynclitic Engagement
Synclitic Engagement

Occurs when the sagittal suture lies exactly in the transverse diameter of the pelvis, ensuring that the fetal head enters the pelvis in a straight alignment.

Asynclitic Engagement

Occurs when the sagittal suture is tilted towards the pubic symphysis or sacral promontory, indicating a lateral deflection of the fetal head. There are two types:

  • Anterior Asynclitism:

    The anterior parietal bone is felt first during per vaginal examination, can also be called Nagliese obliquity, more common in multiparous females, with sagittal suture being deflected towards the sacral promontory. This allows the fetal head to navigate through the pelvis when the pelvic inlet is slightly narrowed.

ANM -> Anterior asynclitism, Nagliese obligatory, More common in Multiparous females.

  • Posterior Asynclitism:

    Posterior parietal bone is felt first during per vaginal examination, also called Litzman obliquity with the sagittal suture deflected towards the pubic symphysis, more common in nulliparous females. This can occur if the sacral promontory is more prominent.

Engaging Diameters

The transverse engaging diameter is the biparietal diameter, measuring 9.5 cm. The anteroposterior (AP) diameter depends on the degree of flexion.

  • Vertex Presentation (Well-flexed): Suboccipitobregmatic diameter (9.5 cm) is the presenting diameter when the head is well-flexed, allowing for the smallest diameter to pass through the pelvis.

  • Deflexed Head: Larger diameters like occipitofrontal present, which can lead to a more difficult delivery.

  • Partially Extended Head (Brow Presentation): Mento vertical diameter presents, often leading to obstructed labor and the need for cesarean section.

  • Fully Extended Head (Face Presentation): Submentobregmatic or submento vertical diameters present. Delivery can sometimes occur vaginally, depending on the pelvic dimensions.

Engagement occurs with marked asynclitism in a platypelloid pelvis, which is characterized by a flattened shape.

The most common cause of a deflexed head in a primigravida female at term is a deflexed head or occipital posterior position, where the fetal head is not properly flexed.

Mechanism of Labor
Initial Position

Lie is longitudinal, presentation is cephalic, presenting part is vertex, denominator is occiput in LOT position. This is the ideal starting position for a smooth vaginal delivery.

Cardinal Movements

  1. Engagement: The biparietal diameter crosses the pelvic inlet, as described earlier.

  2. Descent: The head descends into the pelvis due to uterine contractions and abdominal muscle pressure. This is a gradual process, facilitated by the softening and effacement of the cervix.

  3. Flexion: Occurs simultaneously with descent to present the smallest diameter (suboccipitobregmatic). Flexion is aided by the resistance of the pelvic floor, causing the chin to touch the fetal chest.

Uterine contractions are the primary factor for descent, except in the late stages of labor where maternal pushing efforts dominate. Contractions cause the cervix to dilate and the fetus to move downward.

Relationship Between Occiput and Shoulders

With the occiput in the LOT position, the shoulders are in the anteroposterior diameter. The anterior shoulder is at position three (directly behind the pubic symphysis), and the posterior shoulder is at position seven (directly anterior to the sacral promontory). This alignment is crucial for the shoulders to navigate through the pelvis after the head is delivered.

  1. Internal Rotation: The occiput touches the pelvic floor and rotates anteriorly to lie directly behind the pubic symphysis (DOA position). This occurs due to the elastic recoil of the levator ani muscle (Hart's rule). The occiput rotates by two-eighths of a circle (45 degrees).

  2. Shoulder Rotation: The shoulders rotate only by one-eighth of a circle to reduce torsion on the fetal neck. The shoulders move into the oblique diameter (positions four and eight) or left oblique diameter, aligning them with the widest part of the pelvic outlet.

  3. Crowining: Not a cardinal movement, but the head stretches the perineum permanently as it distends the vaginal opening. This is a critical stage where the fetal head becomes visible and no longer recedes between contractions.

  4. Extension: The head is delivered by extension, with the occiput, vertex, brow, and face emerging sequentially. This occurs as the base of the occiput pivots under the pubic arch.

  5. Restitution: The neck untwists (not a separate cardinal movement; part of external rotation) as the head realigns with the shoulders.

  6. External Rotation: The shoulders rotate back to the AP diameter. This internal rotation of the shoulders manifests as external rotation of the fetal head, aligning it with the shoulders' position.

Final Steps

  1. The anterior shoulder is delivered first, followed by the posterior shoulder. This is typically achieved by gentle downward traction to deliver the anterior shoulder under the pubic arch, followed by upward traction to deliver the posterior shoulder over the perineum.

  2. Expulsion of the body occurs via lateral flexion, as the rest of the body is delivered smoothly after the shoulders.

After external rotation, the occiput returns to its original LOT position, with the face facing towards the mother's right thigh, completing the delivery process.