Overview
This lecture explains the mechanism of labor, detailing the normal criteria, the roles of the passage, passenger, and powers, and the step-by-step cardinal movements of labor, including placental expulsion.
Definition and Criteria of Labor
- Labor is the expulsion of a viable fetus from the uterus through the genital tract.
- WHO defines normal labor as spontaneous onset, low risk throughout, vertex presentation, occurring between 37-42 weeks, vaginal birth, and good maternal and fetal outcomes.
The Three Ps of Labor
- Passage: Includes bony pelvis (assessed by pelvimetry) and soft tissues (lower uterine segment, cervical dilation, vagina).
- Passenger: Refers to the fetus; ideal features are longitudinal lie, vertex presentation, universal flexion, head at station zero or below, and weight 2-3.5 kg.
- Powers: Uterine contractions, abdominal muscles, and maternal effort drive labor.
Cardinal Movements of Labor (in LOA position)
- Engagement: Fetal head’s widest part (biparietal diameter) enters pelvic inlet.
- Descent: Continuous downward movement of the fetus through the pelvis.
- Flexion: Fetal head bends so chin touches chest, changing presenting diameter to the smaller suboccipitobregmatic.
- Internal Rotation: Occiput rotates 45° anteriorly to align with the pelvic outlet.
- Extension: Head extends under the pubic symphysis, allowing delivery of brow, forehead, nose, mouth, and chin.
- Restitution: Head untwists to align with the shoulders after delivery.
- External Rotation: Shoulders rotate to anterior-posterior diameter for birth.
- Expulsion: Rest of the body is delivered easily after shoulders.
Engagement and Asynclitism
- Engagement occurs 2-3 weeks before term in primigravidas, after labor starts in multigravidas.
- Engagement can be assessed abdominally (floating head) or vaginally (presenting part at/below ischial spines).
- Synclitism: Sagittal suture is midway between pubic symphysis and sacral promontory.
- Asynclitism: Sagittal suture is closer to either pubic symphysis (posterior asynclitism/Litzmann) or sacral promontory (anterior asynclitism/Nägele), affecting which parietal bone presents.
Placental Expulsion
- After fetal delivery, uterine contractions and retraction reduce the placental bed.
- Placenta separates along the spongy layer and is expelled vaginally.
- Duncan’s method: Placenta separates from edge, presenting both surfaces together.
- Schultze’s method: Placenta separates from center, presenting the fetal surface first.
Key Terms & Definitions
- Labor — Process of fetal expulsion from uterus.
- Vertex Presentation — Baby’s head-down position, optimal for birth.
- Engagement — Entry of fetal biparietal diameter into pelvic inlet.
- Synclitism — Sagittal suture centered in pelvis.
- Asynclitism — Sagittal suture shifted, leading to anterior or posterior presentation.
- Cardinal Movements — Sequence of positional changes during labor: engagement, descent, flexion, internal rotation, extension, restitution, external rotation, expulsion.
- Restitution — Undoes the twist of the fetal neck after head delivery.
Action Items / Next Steps
- Review diagrams of cardinal movements and pelvis anatomy.
- Memorize the sequence and key features of each cardinal movement.
- Understand the differences and significance of synclitism and asynclitism.