Mechanism of Labour: Physiology
Understanding Labor Mechanism & Signs of Imminent Second Stage
Table of Contents
Introduction to Labor Physiology
Labor is a physiological process that results in the birth of the fetus and expulsion of the placenta. The labor mechanism is a complex series of coordinated events involving hormonal, mechanical, and neurological processes that facilitate the delivery of the baby.
Key Hormones in Labor
- Oxytocin: Primary hormone responsible for uterine contractions
- Prostaglandins: Soften the cervix and enhance uterine contractions
- Relaxin: Relaxes the pelvic joints and softens the cervix
- Endorphins: Natural painkillers released during labor
The labor mechanism is driven by the power of uterine contractions, the passage (birth canal), the passenger (fetus), and positioning. These factors collectively known as the “4 Ps” determine the progression and success of labor.
Memory Aid: The 4 Ps of Labor
Power (contractions) + Passage (birth canal) + Passenger (baby) + Position = Progression of Labor
Stages of Labor
Labor is traditionally divided into three main stages, with the first stage further subdivided into phases.
| Stage | Description | Duration (Primipara) | Duration (Multipara) |
|---|---|---|---|
| First Stage Latent Phase |
From onset of regular contractions to 4cm dilation | Up to 20 hours | Up to 14 hours |
| First Stage Active Phase |
From 4cm to 8cm dilation | ~6 hours | ~4 hours |
| First Stage Transition Phase |
From 8cm to full dilation (10cm) | 1-3 hours | 20 min-1 hour |
| Second Stage | From full dilation to birth of baby | Up to 3 hours | Up to 2 hours |
| Third Stage | From birth of baby to delivery of placenta | 5-30 minutes | 5-30 minutes |
Clinical Pearl: Labor Progress
Remember the “Friedman’s Curve” concept: cervical dilation should progress at approximately 1cm/hour during active labor. Slower progress may indicate dysfunctional labor requiring closer assessment.
The second stage of labor is particularly significant as it represents the time from complete cervical dilation (10cm) to the birth of the baby. During this stage, the labor mechanism becomes most apparent as the fetus navigates through the birth canal.
Mechanism of Labor (Cardinal Movements)
The labor mechanism is a series of positional changes that the fetus undergoes to navigate through the maternal pelvis during childbirth. These movements, known as cardinal movements, occur primarily during the second stage of labor.
Cardinal Movements of Labor
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Engagement: The widest diameter of the presenting part enters the pelvic inlet.
This typically occurs before labor in primigravidas and during labor in multigravidas.
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Descent: The gradual movement of the fetus through the birth canal.
Continues throughout labor but accelerates during the second stage.
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Flexion: The fetal chin tucks onto the chest, presenting the smallest diameter of the head.
This position (occipitoanterior) is optimal for passage through the birth canal.
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Internal Rotation: The fetal head rotates to align with the anteroposterior diameter of the pelvis.
This rotation is critical to navigate the changing pelvic dimensions.
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Extension: As the head reaches the perineum, it extends backward, allowing delivery.
The occiput, brow, and face pass under the pubic symphysis in sequence.
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External Rotation (Restitution): After delivery, the head rotates to align with the shoulders.
This is a passive movement as the shoulders align with the pelvic outlet.
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Expulsion: Delivery of shoulders and the rest of the body.
Anterior shoulder delivers first, followed by posterior shoulder and body.
Memory Aid: “DEFIRE”
Descent, Engagement, Flexion, Internal Rotation, Extension and External Rotation
Clinical Pearl: Cardinal Movements
Although described sequentially, cardinal movements often overlap in practice. For example, descent continues throughout the entire labor process, occurring simultaneously with other movements.
Signs of Imminent Labor in Second Stage
Recognizing the signs of imminent labor in the second stage is crucial for nursing assessment and care planning. These signs indicate that birth is approaching and help guide appropriate interventions.
Physical Signs of Imminent Second Stage Labor
- Bulging Perineum: The perineum becomes distended and thin as the fetal head descends.
- Crowning: The appearance of the fetal head at the vaginal opening during contractions.
- Involuntary Bearing Down/Pushing: The mother experiences an uncontrollable urge to push with contractions.
- Rectal Pressure: Intense pressure in the rectal area as the fetal head descends.
- Purple Line: A line that extends from the anus toward the sacrum, becoming more prominent as labor progresses.
- Anal Pouting/Dilatation: The anus dilates slightly due to pressure from the descending fetal head.
- Increased Bloody Show: More pronounced vaginal bleeding may occur as capillaries rupture during descent.
Maternal Behavioral Signs
- Vocalization Changes: Grunting sounds during contractions or statements like “I need to push” or “The baby is coming.”
- Increased Restlessness: Difficulty finding a comfortable position.
- Change in Facial Expression: Concentrated, focused expression during pushing efforts.
- Loss of Control: May become less responsive to instructions as instinctual urges take over.
- Shaking/Trembling: Involuntary tremors in the legs may occur.
Memory Aid: “CROWNING” Signs
- Contractions intensify
- Rectal pressure increases
- Opening of vagina visible
- Woman feels urge to push
- Noted bulging of perineum
- Increased bloody show
- Noise changes (grunting)
- Glimpse of fetal head
Maternal Physiological Changes During Labor
During labor, particularly as the labor mechanism progresses into second stage, significant physiological changes occur in the maternal body:
| System | Physiological Changes | Clinical Significance |
|---|---|---|
| Cardiovascular |
|
Monitor vital signs; supine position may cause aortocaval compression |
| Respiratory |
|
Support effective breathing patterns; observe for signs of respiratory distress |
| Renal |
|
Empty bladder regularly; full bladder may impede descent |
| Metabolic |
|
Monitor hydration status; provide clear fluids if permitted |
| Gastrointestinal |
|
Position to prevent aspiration if vomiting occurs |
Clinical Pearl: Ferguson’s Reflex
The pressure of the fetal head on pelvic floor receptors stimulates the release of additional oxytocin, intensifying contractions. This neuroendocrine reflex helps coordinate the urge to push with contractions during the second stage of labor.
Nursing Care During Second Stage
Effective nursing care during the second stage of labor supports the normal labor mechanism while ensuring maternal and fetal wellbeing.
Assessment
- Monitor maternal vital signs every 5-15 minutes
- Assess FHR every 5 minutes or continuously
- Evaluate contractions: frequency, duration, intensity
- Assess descent using station measurements
- Monitor for signs of imminent birth
- Evaluate maternal hydration status
- Assess bladder fullness
Support & Interventions
- Provide clear, concise coaching for pushing efforts
- Support preferred positioning for pushing
- Apply warm compresses to perineum
- Provide perineal support as needed
- Keep birth area clean
- Encourage rest between contractions
- Offer positive reinforcement
- Prepare for delivery of infant
Pushing Techniques & Positions
Different pushing techniques and positions can facilitate the labor mechanism and descent of the fetus:
| Position | Benefits | Considerations |
|---|---|---|
| Semi-recumbent | Comfortable, allows for rest between contractions | May reduce pelvic dimensions slightly |
| Lateral/Side-lying | Reduces perineal trauma, improves oxygenation | May slow descent slightly |
| Squatting | Increases pelvic dimensions up to 10%, uses gravity | Physically demanding, requires assistance |
| Hands and knees | Reduces back pain, may assist with rotation | May be difficult to maintain |
| Birthing stool/chair | Utilizes gravity, increases pelvic outlet | Increases risk of perineal edema |
Memory Aid: “SUPPORT” for Second Stage
- Station assessment regularly
- Urge to push with contractions
- Position changes as needed
- Perineal support and protection
- Ongoing encouragement
- Rest between contractions
- Timely assessment of maternal-fetal status
Best Practices & Recent Updates in Labor Management
1. Delayed Pushing in Second Stage
Recent evidence supports allowing a period of passive descent before active pushing when the mother is fully dilated, especially with epidural analgesia.
Clinical Update: The American College of Obstetricians and Gynecologists (ACOG) now supports delaying pushing for 1-2 hours after complete dilation in women with epidurals to reduce the duration of active pushing and decrease maternal exhaustion.
This approach supports the natural labor mechanism by allowing the fetus to rotate and descend passively with uterine contractions before active maternal pushing efforts begin.
2. Physiologic vs. Directed Pushing
Traditional directed pushing (Valsalva maneuver) is being reconsidered in favor of spontaneous pushing.
Clinical Update: Current evidence suggests physiologic pushing (open-glottis pushing in response to urges) may reduce maternal fatigue and fetal heart rate abnormalities compared to directed closed-glottis pushing.
This approach aligns with the natural labor mechanism and maternal instincts, potentially reducing the risk of pelvic floor trauma.
3. Upright Positioning During Second Stage
Traditional supine positioning is being replaced by more upright positions in many settings.
Clinical Update: The World Health Organization recommends that women adopt the position they find most comfortable during the second stage. Upright positions (standing, squatting, kneeling) may shorten the second stage and reduce instrumental deliveries.
Upright positions utilize gravity to facilitate the labor mechanism and may increase pelvic dimensions by up to 30% compared to supine positions.
Potential Complications During Second Stage
While the labor mechanism typically progresses normally, nurses must be alert to potential complications that may disrupt the process:
| Complication | Signs/Symptoms | Nursing Interventions |
|---|---|---|
| Prolonged Second Stage |
|
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| Fetal Distress |
|
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| Shoulder Dystocia |
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| Maternal Exhaustion |
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Clinical Pearl: Preventing Perineal Trauma
Warm compresses applied to the perineum during the second stage have been shown to reduce the risk of third and fourth-degree lacerations by up to 44%, according to systematic reviews. This simple intervention supports optimal tissue elasticity during the final phases of the labor mechanism.
Summary
Understanding the labor mechanism and recognizing signs of imminent second stage labor are essential nursing competencies in obstetrical care. This knowledge enables nurses to:
- Anticipate normal labor progression and identify deviations
- Provide appropriate support during the cardinal movements
- Recognize when birth is imminent
- Implement evidence-based practices to support physiological birth
- Collaborate effectively with the interdisciplinary team
- Document labor progress accurately and comprehensively
By integrating an understanding of labor physiology with current evidence-based practices, nurses can promote optimal outcomes for mothers and newborns while respecting the natural birth process.
References
- American College of Obstetricians and Gynecologists. (2019). Approaches to limit intervention during labor and birth. ACOG Committee Opinion No. 766. Obstetrics & Gynecology, 133(2), e164-e173.
- Bohren, M. A., Hofmeyr, G. J., Sakala, C., Fukuzawa, R. K., & Cuthbert, A. (2017). Continuous support for women during childbirth. Cochrane Database of Systematic Reviews, 7, CD003766.
- Cunningham, F. G., Leveno, K. J., Bloom, S. L., Dashe, J. S., Hoffman, B. L., Casey, B. M., & Spong, C. Y. (2018). Williams Obstetrics (25th ed.). McGraw-Hill Education.
- Lowdermilk, D. L., Perry, S. E., Cashion, K., & Alden, K. R. (2020). Maternity and women’s health care (12th ed.). Elsevier.
- World Health Organization. (2018). WHO recommendations: intrapartum care for a positive childbirth experience. World Health Organization.
- Simpson, K. R., & O’Brien-Abel, N. (2021). AWHONN’s Perinatal Nursing (5th ed.). Wolters Kluwer.
- Hanson, L., & VandeVusse, L. (2014). Supporting labor progress toward physiologic birth. The Journal of Perinatal & Neonatal Nursing, 28(2), 101-107.
- Aasheim, V., Nilsen, A. B., Lukasse, M., & Reinar, L. M. (2017). Perineal techniques during the second stage of labour for reducing perineal trauma. Cochrane Database of Systematic Reviews, 6, CD006672.