5.2 Second Stage Management, Mechanisms of Normal Labour & Perineal Protection

Second Stage Management, Mechanisms of Normal Labour & Perineal Protection

The second stage of labour represents the culmination of uterine contractions, maternal expulsive efforts, and complex fetal navigational mechanisms resulting in the birth of the baby. For registered midwives, managing this stage demands a sophisticated understanding of pelvic biomechanics, continuous fetal surveillance, and sensitive clinical coaching that empowers maternal autonomy while safeguarding the perineum from avoidable trauma.

---

Definition, Phases and Timeframes of the Second Stage

Under NICE guideline NG235, the second stage of labour is defined as the interval from the confirmed complete dilatation of the cervix (10 cm) to the complete birth of the baby. Clinically and physiologically, the second stage is divided into two distinct phases:

1. The Passive Second Stage

The passive second stage occurs when the cervix has achieved full dilatation (confirmed by vaginal examination or verified by unequivocal clinical signs), but the fetal presenting part has not yet descended deep into the pelvic floor. In this phase:

• The woman does not yet experience an involuntary, overwhelming urge to bear down.
• Uterine contractions may transiently space out, diminish in intensity, or cease temporarily—a phenomenon historically termed the "rest and be thankful" phase.
• This period allows passive fetal descent and internal rotation through continued involuntary myometrial contraction and retraction while preserving maternal energy.
• Management: Midwives should support this physiological resting phase rather than instructing the woman to engage in premature, directed pushing (Valsalva pushing). If a woman without an epidural is fully dilated but has no urge to push, NICE NG235 advises offering further assessment after 1 hour. With an epidural, delayed directed pushing (up to 2 hours after full dilatation for nulliparous women, and 1 hour for multiparous women) may shorten the active second stage.

2. The Active Second Stage

The active second stage commences when:

• The fetal presenting part is visible at the introitus; or
• Full cervical dilatation is accompanied by spontaneous, involuntary expulsive contractions; or
• The woman initiates active maternal pushing efforts under an overwhelming, reflex expulsive urge.

This expulsive reflex occurs as the advancing fetal head stretches the sensory mechanoreceptors in the levator ani muscle sling and the rectal vault, triggering the neurohumoral Ferguson reflex and stimulating the somatic pudendal reflex.

               PHASES OF THE SECOND STAGE OF LABOUR
   Full Dilatation of Cervix (10 cm) Confirmed / Identified
                             │
                             ▼
                 PASSIVE SECOND STAGE
    - No involuntary maternal expulsive urge
    - Presenting part descending passively onto pelvic floor
    - Uterine contractions may space out; maternal rest supported
                             │
                             ▼ (Descent triggers Ferguson reflex)
                 ACTIVE SECOND STAGE
    - Overwhelming, involuntary bearing-down urges
    - Presenting part visible at the vulva
    - Active maternal expulsive efforts
                             │
                             ▼
                    Birth of the Infant

Expected Durations and Clinical Thresholds for Delay

NICE intrapartum guidelines establish clear evidence-based thresholds for normal duration and diagnosis of delay in the active second stage:

Women with an epidural follow the same active-stage expectations, but may already have had a passive stage of up to 2 hours (nulliparous) or 1 hour (multiparous) before active pushing began.

---

Mechanisms of Normal Labour (Vertex Presentation, LOA)

For a fetus presenting by the vertex in an occipito-anterior position—most commonly Left Occipito-Anterior (LOA)—delivery requires the fetal head and trunk to navigate the curves and varying dimensions of the maternal bony pelvis. Because the pelvic brim is widest in its transverse diameter (approx. 13 cm) while the pelvic outlet is widest in its anteroposterior diameter (approx. 13 cm), the fetus must undergo a series of coordinated passive movements known as the cardinal mechanisms of labour.

1. Descent

Descent is the continuous prerequisite for all other movements. It is propelled by four distinct forces: pressure of the amniotic fluid; direct downward pressure of the fundus against the fetal breech during contractions; contraction of the abdominal wall and diaphragm during maternal expulsive efforts; and progressive extension and straightening of the fetal body.

2. Flexion

As the descending fetal head encounters mechanical resistance from the lower uterine segment, the pelvic sidewalls, and the muscular pelvic floor (levator ani), the head flexes against the fetal chest. Flexion acts as a critical biomechanical adaptation:

• It substitutes the smaller suboccipitobregmatic diameter (9.5 cm) and suboccipitofrontal diameter (10.0-10.5 cm) for the much larger occipitofrontal diameter (11.5 cm).
• This presents the smallest possible cephalic perimeter to the pelvic canal, facilitating easier passage through the pelvic planes.

3. Internal Rotation of the Head

The pelvic floor forms a V-shaped muscular gutter sloping downward and forward, formed by the pubococcygeus and iliococcygeus muscles of the levator ani. According to Sellheim's law, a cylindrical body forced through a curved, unequal cylinder rotates so that its most mobile part adapts to the greatest curve.

• In LOA, the leading, lowest-descending point is the occiput.
• Upon striking the left slope of the pelvic floor, the occiput rotates 1/8 of a circle (45 degrees) anteriorly, moving from the left oblique diameter to lie directly beneath the symphysis pubis (anteroposterior diameter of the pelvic outlet).
• Internal rotation twists the fetal neck by 45 degrees, as the shoulders remain in the oblique diameter at the pelvic brim.

4. Extension

Once the rotated occiput escapes the subpubic arch, the suboccipital region (the nape of the neck) impinges against the lower border of the symphysis pubis, functioning as a fixed mechanical fulcrum (pivot point).

• Because the pelvic outlet curves upward and forward (the curve of Carus), the head cannot descend further in flexion.
• Forced forward by myometrial contractions and supported by the elastic pelvic floor, the head pivots around the subpubic fulcrum and delivers by extension.
• The bregma, forehead, nose, mouth, and chin sweep successively over the anterior perineum, freeing the head from the vulva.

5. Restitution

The moment the fetal head is born and released from the pressure of the vulval tissues, the twisted fetal neck untwists spontaneously:

• The occiput rotates 1/8 of a circle (45 degrees) back towards the maternal left side (in an LOA mechanism).
• This movement restores the head to its natural right-angle alignment with the fetal bisacromial diameter (shoulders), which are currently engaging in the oblique diameter of the pelvis.

6. External Rotation of the Shoulders and Head

The fetal shoulders now descend into the pelvic cavity. Meeting the sloping gutter of the pelvic floor, the anterior shoulder rotates anteriorly 1/8 of a circle (45 degrees) to lie directly behind the symphysis pubis, bringing the bisacromial diameter into the direct anteroposterior diameter of the outlet.

• Simultaneously, this shoulder rotation is transmitted to the delivered fetal head outside the vulva.
• The occiput rotates a further 1/8 of a circle (45 degrees) in the same direction as restitution, so that the fetal face turns completely towards the maternal right thigh (in LOA, looking directly towards the maternal right medial thigh).

7. Lateral Flexion and Expulsion

Once the bisacromial diameter lies in the anteroposterior diameter of the outlet:

• The anterior shoulder impinges beneath the subpubic arch.
• Under physiological maternal effort (or gentle downward guidance by the midwife), the anterior shoulder is released beneath the pubic bone.
• The midwife then guides the fetal head gently upward towards the maternal abdomen following the curve of Carus, allowing the posterior shoulder to distend the perineum and sweep over the perineal body.
• The remainder of the fetal trunk and limbs is expelled rapidly by lateral flexion of the fetal spine, accommodating the natural curve of the maternal pelvis.

---

Intermittent Auscultation Protocol During the Second Stage

The second stage of labour carries an elevated risk of fetal hypoxaemia and acidaemia. Prolonged, intense myometrial contractions significantly reduce uteroplacental blood flow, while active maternal bearing-down efforts increase intra-abdominal pressure and intermittent umbilical cord compression.

Auscultation Schedule and Technique

Under NICE guideline NG229 (Fetal monitoring in labour):

• Frequency: The fetal heart rate must be auscultated at least every 5 minutes throughout the active second stage (or after every contraction if expulsive contractions are occurring with high frequency).
• Duration: The heart rate must be counted for one full minute immediately following an expulsive contraction.
• Method: Using a Pinard stethoscope or handheld Doppler ultrasound, while simultaneously palpating the maternal radial pulse to ensure differentiation.
• Assessment: Verify that the baseline heart rate remains between 110 and 160 bpm and that there is a prompt, crisp return to baseline without persistent audible decelerations.

---

Maternal Positioning and Pelvic Biomechanics

Evidence-informed midwifery care recognizes that maternal posture during the second stage profoundly alters maternal hemodynamics, pelvic diameters, and labour progress.

Physiological vs Pathological Positions

                     PELVIC BIOMECHANICS & POSITIONING
  ┌─────────────────────────────────────┬─────────────────────────────────────┐
  │     UPRIGHT & MOBILE POSTURES       │       SUPINE / DORSAL POSTURES      │
  │  (Squatting, All-Fours, Kneeling)   │        (Flat Supine, Lithotomy)     │
  ├─────────────────────────────────────┼─────────────────────────────────────┤
  │ • Pelvic outlet widens by up to 30% │ • Sacrum pinned against bed matrix  │
  │ • Gravity assists fetal descent     │ • Pelvic outlet narrowed & rigid    │
  │ • Aortocaval compression avoided    │ • Aortocaval compression (Poseiro)  │
  │ • Shorter active pushing duration   │ • Fetal hypoxaemia & maternal drop  │
  │ • Lower rates of severe tears/OASIS │ • Higher rate of episiotomy/forceps │
  └─────────────────────────────────────┴─────────────────────────────────────┘

Upright, Mobile, and Lateral Postures

• Upright Postures (Walking, Standing, Supported Squatting, Kneeling): Upright and forward-leaning positions maximize the gravitational vector, aiding fetal descent. Crucially, allowing the maternal sacrum to pivot freely at the sacroiliac and sacrococcygeal joints increases the dimensions of the pelvic outlet; radiological studies of squatting reported up to a 28% increase in outlet area compared with supine postures.
• All-Fours Position (Hands and Knees): Highly effective for relieving severe sacral back pain, encouraging rotation of persistent occipito-posterior (OP) and occipito-transverse (OT) fetal malpositions, and distributing perineal tension symmetrically.
• Left Lateral Position: Ideal when the woman requires rest, has sustained fatigue, or has an epidural. It prevents aortocaval compression while maintaining optimal pelvic mobility and perineal visibility.

Hazards of the Supine and Dorsal Lithotomy Positions

Midwives actively discourage women from assuming flat supine or semi-recumbent positions with knees pulled back (lithotomy):

• Aortocaval Compression (The Poseiro Effect): The heavy gravid uterus compresses the inferior vena cava and descending abdominal aorta against the lumbar spine. This reduces venous return to the maternal right atrium, causing a precipitous drop in cardiac output and maternal hypotension, leading to acute reductions in uteroplacental perfusion and fetal bradycardia.
• Pelvic Outlet Restriction: Lying flat on the sacrum pins the coccyx against the bed, eliminating sacrococcygeal mobility and artificially reducing the functional anteroposterior diameter of the pelvic outlet.
• Increased Perineal Trauma: Supine and lithotomy positions place uneven, excessive tension across the posterior perineal body, significantly increasing rates of episiotomy, instrumental delivery, and severe third- and fourth-degree perineal tears.

---

Perineal Protection Techniques and Evidence-Based Practice

Preserving perineal integrity and preventing obstetric anal sphincter injuries (OASIS)—which encompass third- and fourth-degree perineal tears—is a primary clinical focus of the second stage.

"Hands Poised" vs "Hands On"

Controversy historically surrounded the comparative efficacy of "hands poised" (hands off) versus "hands on" at the crowning of the fetal head:

• Hands Poised: The midwife keeps their hands poised close to the perineum without touching the fetal head or perineal body until delivery, allowing tissues to distend naturally.
• Hands On: The midwife places the finger pads of one hand gently against the advancing fetal occiput to maintain flexion and regulate the speed of crowning, while the other hand supports the perineum with a warm sterile compress.
• Evidence: Major randomized trials (such as the HOOP trial) demonstrated comparable rates of perineal trauma between both approaches. Modern UK guidelines (including RCOG and NICE) emphasize that midwives should adapt their technique to maternal preference and clinical circumstances, focusing primarily on controlling the speed of crowning.

Warm Perineal Compresses

Applying a warm, moist compress (gauze or cloth soaked in warm tap water, wrung out and applied directly to the perineal body during contractions in the active pushing phase) is strongly supported by Cochrane systematic reviews. Warm compresses:

• Increase local tissue perfusion, elasticity, and muscle relaxation.
• Provide comforting non-pharmacological sensory relief.
• Demonstrate a statistically significant reduction in the incidence of third- and fourth-degree perineal tears (OASIS).

Breathing the Head Out

As the fetal head crowns (when the widest biparietal diameter distends the vulval introitus without receding between contractions), forceful maternal bearing down significantly increases perineal shearing forces. The midwife must clearly coach the woman to:

• Stop active pushing or bearing down;
• Take shallow breaths, pant, or blow out gently;
• Allow the head to ease out slowly between contractions under residual uterine tone alone.

Restrictive Episiotomy vs Routine Episiotomy

Routine episiotomy is strongly contraindicated in evidence-based practice. NICE and RCOG guidelines mandate a restrictive episiotomy policy:

• Indications: Episiotomy is performed only for recognized clinical indications, including: suspected fetal distress requiring expedited birth; instrumental vaginal delivery (particularly forceps); or imminent severe, complicated perineal breakdown.
• Technique: When an episiotomy is indicated, it must be performed as a mediolateral episiotomy. The incision begins at the posterior fourchette in the midline and is angled laterally at 45 to 60 degrees from the vertical midline when the perineum is fully distended. This angled incision directs tissue disruption away from the anal sphincter complex, protecting against rectal buttonhole tears and complete sphincter disruption.