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Mechanisms of Labor | 마이메르시 MyMerci
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Mechanisms of Labor

NCLEX Review Guide: Mechanisms of Labor

Cardinal Movements of Labor

The Seven Cardinal Movements

  • Engagement: The fetal presenting part enters the pelvic inlet, with the biparietal diameter passing through the pelvic inlet (occurs at 0 station).
  • Descent: Progressive movement of the fetus through the birth canal, measured by fetal station from -5 to +5.
  • Flexion: The fetal head flexes so the chin touches the chest, presenting the smallest diameter (suboccipitobregmatic) to the pelvis.
  • Internal Rotation: The fetal head rotates from transverse to anteroposterior position to accommodate the pelvic outlet.
  • Extension: The fetal head extends as it passes under the symphysis pubis, allowing the occiput, brow, face, and chin to emerge.
  • External Rotation (Restitution): The head rotates back to its natural alignment with the shoulders after delivery.
  • Expulsion: Delivery of the anterior shoulder, then posterior shoulder, followed by the rest of the fetal body.

Memory Aid: "Every Darn Fool In Egypt Eats Eggs"

Engagement → Descent → Flexion → Internal Rotation → Extension → External Rotation → Expulsion

Key Points

  • Station 0 = engagement; +5 = crowning
  • Flexion reduces the presenting diameter from 13.5cm to 9.5cm
  • Internal rotation is essential for vaginal delivery

Fetal Positioning and Presentation

Key Terminology

  • Lie: Relationship of fetal spine to maternal spine (longitudinal, transverse, or oblique).
  • Presentation: Fetal part that enters the pelvis first (vertex, breech, or shoulder).
  • Position: Relationship of fetal presenting part to maternal pelvis (LOA, ROA, LOP, ROP).
  • Attitude: Degree of fetal head flexion (flexed, military, or extended).

Vertex Positions Comparison

PositionLocationLabor Progress
LOA (Left Occiput Anterior)Most favorableFastest labor
ROA (Right Occiput Anterior)Second most favorableNormal progress
LOP/ROP (Posterior)Occiput toward sacrumProlonged labor, back pain

Clinical Scenario

A laboring client reports severe back pain with contractions. Fetal heart tones are heard best at the umbilicus level, slightly to the right. This suggests ROP position - prepare for longer labor and increased pain management needs.

Factors Affecting Labor Mechanisms

The 5 P's of Labor

  1. Passenger: Fetal size, position, and presentation affect mechanism progression
  2. Passageway: Maternal pelvis size and shape determine accommodation
  3. Powers: Uterine contractions and maternal pushing efforts drive descent
  4. Position: Maternal positioning affects pelvic dimensions and fetal rotation
  5. Psychological: Maternal anxiety and fear can inhibit labor progress

Key Points

  • Gynecoid pelvis is most favorable for vaginal delivery
  • Macrosomia (>4000g) increases risk of shoulder dystocia
  • Maternal positioning can increase pelvic outlet by 1-2 cm

Common Complications and Nursing Interventions

Abnormal Labor Patterns

  • Arrest of Descent: No progress for 1 hour (nullipara) or 30 minutes (multipara) during active pushing - consider operative delivery.
  • Shoulder Dystocia: Anterior shoulder impacted behind symphysis pubis - perform McRoberts maneuver and suprapubic pressure.
  • Asynclitism: Lateral flexion of fetal head causing uneven descent - position changes may help correction.

Shoulder Dystocia Management: "HELPERR"

Help - call for assistance
Evaluate for episiotomy
Legs - McRoberts position
Pressure - suprapubic
Enter - internal maneuvers
Remove - posterior arm
Roll - to all fours

Key Points

  • Never apply fundal pressure during shoulder dystocia
  • Document exact time of shoulder dystocia occurrence
  • Monitor for postpartum hemorrhage after difficult delivery

Quick Check & Common Pitfalls

Self-Assessment

Quick Check Questions

Can you name all 7 cardinal movements in order?
Do you know the difference between engagement and descent?
Can you identify favorable vs. unfavorable fetal positions?
Do you understand the 5 P's and their interactions?

Common Pitfalls to Avoid

  • Confusing engagement (entering pelvis) with crowning (visible at perineum)
  • Forgetting that posterior positions cause prolonged labor and back pain
  • Not recognizing that station +5 means birth is imminent
  • Mixing up internal rotation (fetus rotates) with external rotation (restitution)

Remember: Understanding labor mechanisms helps you anticipate complications and provide optimal nursing care. You're preparing to be an amazing labor and delivery nurse - trust your knowledge and clinical judgment!

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