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Mechanism of Normal Labour

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Prerequisites

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Stages of Labour (Overview)

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Cardinal Movements of Labour (Second Stage)

1. Engagement

• The biparietal diameter (BPD) passes through the pelvic inlet (plane of the brim).
• The occiput descends to or below the level of the ischial spines (station 0).
• In primigravidae, engagement occurs at 36–38 weeks; in multigravidae, often only in labour.
• The head enters in the transverse or oblique diameter of the pelvis.

2. Descent

• The most continuous movement throughout labour.
• Occurs due to: uterine contractions, bearing-down efforts, straightening of the fetal body, and fundal pressure.
• Assessed clinically by fifths of the head palpable abdominally and station on vaginal examination.

3. Flexion

• As the head descends and meets pelvic floor resistance, it flexes fully.
• The chin is brought to the chest, converting the presenting diameter from the occipito-frontal (~11.5 cm) to the suboccipito-bregmatic (~9.5 cm) — the smallest anteroposterior diameter.
• This is a passive movement facilitated by the levator ani muscles.

4. Internal Rotation

• The occiput rotates anteriorly from the transverse/oblique position to the direct occiput anterior (OA) position under the symphysis pubis.
• Rotation is 45° from LOA or 90° from LOT to OA.
• Caused by the gutter shape of the levator ani directing the presenting part forward.
• The occiput aligns with the anteroposterior diameter of the pelvic outlet.

5. Crowning

• The head no longer recedes between contractions.
• The suboccipital region (nape of neck) pivots under the symphysis pubis (hypomochlion).
• The widest diameter of the head (BPD) distends the vulva.

6. Extension

• The flexed head extends around the symphysis pubis.
• The vertex, bregma, forehead, face, and chin deliver in succession over the perineum.
• Driven by the combined forces of uterine contractions and the resistance of the pelvic floor.
• The baby is born face down (facing the maternal anus).

7. Restitution

• After the head is delivered, it rotates 45° back to its natural alignment with the shoulders (undoing internal rotation).
• The occiput turns to the left (in LOA) — the head aligns with the shoulders, which are still in the oblique diameter.

8. External Rotation (Second Internal Rotation of Shoulders)

• The shoulders undergo internal rotation: the anterior shoulder rotates under the symphysis pubis (aligning shoulders to the AP diameter).
• The head simultaneously rotates a further 45° in the same direction — occiput moves to the left transverse position.
• This is visible externally as further head rotation after restitution.

9. Expulsion (Delivery of the Body)

• Anterior shoulder delivers from under the symphysis pubis first (with lateral flexion toward the perineum).
• Posterior shoulder sweeps over the perineum.
• The rest of the body delivers by lateral flexion.

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Summary Table of Cardinal Movements

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First Stage: Mechanism at the Cervix

• Latent phase: 0–4 cm dilatation; slow progress; cervical effacement occurs.
• Active phase: 4–10 cm; rate ≥1 cm/hour (WHO 2018 threshold); faster progress.
• Cervical dilatation driven by: uterine contractions, the bag of membranes (hydrostatic wedge), and the descending presenting part.

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Third Stage: Placental Delivery

• Calkin's sign: uterus becomes globular and firm
• Cord lengthening (Küstner/Strassmann sign)
• Suprapubic gush of blood
• Uterus rises in the abdomen

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Key Diameters of the Fetal Head

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Clinical Mnemonics

Engagement → Descent → Flexion → Internal rotation → Restitution → Extension → External rotation (+ Expulsion)

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Ultrasound in Pregnancy — Comprehensive Guide

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Overview: Routine Scans in Pregnancy

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1. Early Viability Scan (6–8 Weeks)

Key Findings:

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2. Dating / Nuchal Translucency Scan (11–13+6 Weeks)

Gestational Age Dating:

• CRL 45–84 mm — accurate to ±5 days
• If CRL >84 mm (>14 weeks), use head circumference (HC) for dating
• Supersedes LMP if discrepancy >7 days (first trimester) or >14 days (second trimester)

Nuchal Translucency (NT):

Combined First Trimester Screening:

• NT + maternal serum PAPP-A + free β-hCG + maternal age
• Detection rate: ~85–90% for T21 with 5% false positive rate

Additional First Trimester Findings:

• Number of fetuses, chorionicity (in twins: lambda vs. T-sign)
• Uterine anatomy (fibroids, septa)
• Ovarian pathology (corpus luteum cyst)
• Subchorionic hematoma
• Crown-rump length measurement
• Fetal heart rate assessment

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3. Anomaly Scan (18–20 Weeks)

A. Head and Brain

B. Face

C. Neck

• Nuchal edema / cystic hygroma
• Neck masses (teratoma, goiter)

D. Spine

• Sagittal view: Continuity of vertebral bodies and posterior elements; "railway track" pattern
• Transverse view: Three ossification centres in each vertebra; intact skin covering
• Spina bifida: Open defect with absent/disrupted posterior elements

E. Thorax and Heart

• Heart occupies ~1/3 of chest area
• Apex points left, 4-chamber axis ~45°
• No cardiomegaly (cardiac: thoracic ratio <0.5 by area)
• Normal rhythm; rate 120–160 bpm

• Homogeneous echogenicity
• Echogenic mass → CPAM (congenital pulmonary airway malformation) or sequestration

F. Abdomen

G. Limbs

• Long bones measured: femur length (FL), humerus length (HL)
• All four limbs, three segments each (proximal/mid/distal)
• Hands and feet: presence, position (clubfoot, rocker-bottom foot)
• Skeletal dysplasia: short, bowed, or fractured bones

H. Soft Markers for Aneuploidy

I. Placenta and Liquor

• Placental site: anterior, posterior, fundal, lateral; low-lying if <2 cm from internal os
• Placental texture: homogeneous (Grade 0 at 18–20 weeks)
• Amniotic fluid index (AFI): 5–24 cm (normal); or deepest vertical pool (DVP) 2–8 cm
• Cervical length: measured transvaginally; <25 mm = short cervix, risk of PTB

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4. Growth Scans (28, 32, 36 Weeks)

Biometric Parameters:

Interpretation:

Placental Grading (Grannum):

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5. Doppler Studies

Uterine Artery Doppler (11–13 weeks and 20–24 weeks):

• Normal: Low-resistance waveform; RI <0.58, PI <1.45
• Abnormal: High resistance, notching (bilateral > unilateral)
• Predicts pre-eclampsia and FGR

Umbilical Artery (UA) Doppler:

Middle Cerebral Artery (MCA) Doppler:

• Brain-sparing: MCA PI decreases (vasodilation) in response to hypoxia
• Cerebro-placental ratio (CPR) = MCA PI / UA PI; <1.0 indicates redistribution
• MCA PSV: Used to detect fetal anaemia (>1.5 MoM = significant anaemia)

Ductus Venosus (DV) Doppler:

• Reflects right heart filling pressure / venous pressure
• Absent or reversed a-wave = advanced cardiac compromise; imminent fetal death
• Triggers delivery decision in severe FGR

Sequence of Doppler Changes in FGR:

Elevated UA PI → AEDF → REDF
       ↓
Decreased MCA PI (brain-sparing)
       ↓
Decreased CPR
       ↓
Abnormal DV (absent/reversed a-wave)
       ↓
Abnormal CTG → Fetal death

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6. Biophysical Profile (BPP)

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The Five Components (Manning's BPP)

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Physiological Basis of BPP

NST reactivity → Fetal breathing → Body movements → Fetal tone

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BPP Score Interpretation and Management

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Modified BPP (mBPP)

• NST (acute marker) + AFI (chronic marker)
• Normal: Reactive NST + AFI ≥5 cm
• Abnormal: Non-reactive NST or AFI <5 cm → full BPP or delivery decision
• Sensitivity ~90% with less time required

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Limitations of BPP

• False positives: fetal sleep cycles (20–40 min), maternal sedation, CNS depressants, prematurity (FBM less regular <26 weeks)
• False negatives: rare but possible in acute events
• Requires 30-minute observation window (time-consuming)
• Operator-dependent

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BPP vs. NST vs. CST Comparison

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7. Special Ultrasound Assessments

Cervical Length (TVS)

• Measured transvaginally at 16–24 weeks
• Normal: >25 mm
• Short cervix: <25 mm → risk of preterm labour; consider cervical cerclage or progesterone
• Funnelling: Internal os dilatation; U-shaped or V-shaped

Placenta Praevia

• Low-lying placenta: lower edge <20 mm from internal os at term
• Major praevia: Placenta covers internal os
• TVS most accurate for assessment
• Anterior low-lying with previous CS → suspect placenta accreta spectrum

Multiple Pregnancy Ultrasound

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Summary: Key Ultrasound Parameters at a Glance

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