Normal Labour & Mechanism of Labour

Labour is the physiological process by which the products of conception (fetus, placenta, membranes) are expelled from the uterus after the 28th week of gestation. This chapter integrates the stages of labour, the cardinal movements of an occipito-anterior mechanism, partographic monitoring, engagement, and station assessment — a perennial high-yield zone for NEET PG, tested in both diagram-recall and clinical-vignette formats.

Definition & basic terminology

Labour = onset of regular, painful uterine contractions accompanied by progressive cervical effacement and dilatation, with descent of the presenting part. Mere contractions without cervical change = false labour.

• Normal (eutocia) labour: spontaneous onset, vertex presentation, low risk at start and throughout, term gestation (37–42 weeks), spontaneous vaginal delivery within a reasonable time, no complications.
• Lightening: descent of fundal height ~2 weeks before labour in primigravidas (engagement of head).
• Show: passage of blood-stained mucus (expulsion of cervical mucus plug/operculum) — a sign labour is imminent.
• True vs false labour is a favourite one-liner table:

Feature	True labour	False labour
Contractions	Regular, increasing intensity & frequency	Irregular, no progression
Pain site	Back → abdomen (fundal dominance)	Lower abdomen/groin only
Cervical change	Progressive effacement + dilatation	None
Show	Present	Usually absent
Effect of sedation	Not relieved	Often relieved

High-yield: The single defining criterion of true labour is progressive cervical dilatation with descent, not the contractions themselves.

Causation of labour onset (brief pathophysiology)

The exact trigger is multifactorial. Key tested mediators:

• Prostaglandins (PGE₂, PGF₂α): the final common pathway — cause cervical ripening and myometrial contraction; PGE₂ (dinoprostone) is used clinically for ripening.
• Oxytocin & oxytocin receptors: receptor density rises dramatically near term; oxytocin acts on Gq → IP₃ → intracellular Ca²⁺ rise.
• Oestrogen:Progesterone ratio shift: rising oestrogen increases gap junctions (connexin-43) and oxytocin receptors; functional progesterone withdrawal.
• Fetal cortisol surge (CRH–ACTH–cortisol axis) increases oestrogen and prostaglandin synthesis.
• Ferguson reflex: stretch of cervix/upper vagina → reflex oxytocin release (neuroendocrine positive feedback).

High-yield: Prostaglandins are the final common mediator of cervical ripening; functional progesterone withdrawal + rising gap junctions make the myometrium a coordinated syncytium.

Stages of labour

Labour is classically divided into four stages.

Stage	Definition	Duration (primi)	Duration (multi)
First	Onset of true labour → full dilatation (10 cm)	~8–12 h (up to 12–14)	~6–8 h
Second	Full dilatation → delivery of baby	~1–2 h (up to 3 with epidural)	~30 min–1 h
Third	Delivery of baby → expulsion of placenta	5–15 min (≤30 min)	5–15 min
Fourth	First 1 hour postpartum (observation for PPH)	1 h	1 h

First stage — two phases

1. Latent phase: onset of labour → ~4 cm (older WHO) or 5 cm (revised WHO/ACOG 2014) dilatation. Slow dilatation, cervix effaces.

2. Active phase: rapid dilatation. Traditional Friedman cut-off for active phase onset was 3–4 cm; the revised standard now places active phase onset at 6 cm (Zhang's data), which has reduced "failed progress" caesareans.

High-yield (recently re-emphasised): Per revised ACOG/WHO criteria, the active phase begins at 6 cm, and arrest of dilatation should be diagnosed only at ≥6 cm with ruptured membranes and no cervical change for ≥4 hours despite adequate contractions (or ≥6 h with inadequate contractions).

• Friedman's classic minimal active-phase dilatation rate: 1.2 cm/h (primi), 1.5 cm/h (multi).
• Second stage arrest: no descent/rotation after 2 h pushing in multi, 3 h in primi (add 1 h each if epidural in situ).

The PASSAGES, PASSENGER and POWERS

The "3 P's" determine labour outcome: Power (uterine contractions + maternal effort), Passage (bony pelvis + soft tissues), Passenger (fetus — lie, presentation, attitude, position, size).

Fetal head landmarks (must memorise for mechanism)

• Suboccipitobregmatic (SOB) diameter = 9.5 cm — engaging diameter in a well-flexed vertex (occipito-anterior). The smallest AP diameter.
• Occipitofrontal = 11.5 cm (deflexed head/OP).
• Mentovertical = 14 cm — largest; presents in brow presentation (hence brow usually undeliverable vaginally at term).
• Submentobregmatic = 9.5 cm — engaging diameter of face (mentoanterior) presentation.
• Biparietal diameter (BPD) = 9.5 cm; bitemporal = 8 cm.

High-yield: A well-flexed vertex engages with the suboccipitobregmatic 9.5 cm. Brow (mentovertical 14 cm) is the least favourable vertex variant.

Engagement & station

• Engagement: passage of the widest transverse diameter (biparietal) of the head through the pelvic inlet. Clinically corresponds to the head being felt ≤2/5ths abdominally (3/5 or more "fixed").
• In primigravidas, engagement usually occurs 2–3 weeks before term; in multigravidas, often only after labour onset.
• Station: level of the lowest bony point of the presenting part relative to the ischial spines (the maternal reference). Spines = station 0. Measured –5 to +5 cm (or –3 to +3 in fifths). Station 0 = engaged.
• Synclitism/asynclitism: relation of sagittal suture to the inlet; anterior asynclitism (Naegele) and posterior asynclitism (Litzmann).

Mechanism of labour (cardinal movements)

For the commonest scenario — vertex, left occipito-anterior (LOA) — the cardinal movements occur sequentially and somewhat simultaneously. The classic mnemonic is "Every Decent Family Invites Examiners Round Especially Easter":

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

Stepwise flow:

1. Engagement — biparietal diameter passes the inlet; head usually enters in the transverse or oblique diameter of the inlet (transverse diameter of inlet is widest, 13 cm).
2. Descent — continuous; driven by uterine contractions, maternal pushing (after full dilatation), and fetal axis pressure. The only movement occurring throughout.
3. Flexion — as the head meets pelvic floor resistance, the chin is driven onto the chest, converting the presenting diameter from occipitofrontal (11.5 cm) to suboccipitobregmatic (9.5 cm).
4. Internal rotation — the occiput rotates anteriorly by 45° (from LOA to OA) to lie under the symphysis, guided by the levator ani / pelvic floor gutter (forward and medial slope). Occurs at the level of the ischial spines (mid-pelvis).
5. Extension (crowning then delivery) — the flexed head, now beneath the pubic arch, delivers by extension, pivoting around the suboccipital region against the inferior margin of symphysis (the fulcrum). Sequentially the bregma, forehead, nose, mouth and chin sweep over the perineum.
6. Restitution — the delivered head untwists 45° to realign with the shoulders (which are still in oblique).
7. External rotation — shoulders rotate internally to AP diameter of outlet; the head externally rotates a further 45° (occiput now points to maternal thigh).
8. Expulsion — anterior shoulder delivers under the symphysis first, then posterior shoulder over perineum, then the trunk.

High-yield: Internal rotation is brought about by the shape and slope of the pelvic floor (levator ani) and occurs at the level of the ischial spines. Descent is the only movement continuous throughout labour.

High-yield: The fulcrum/pivot for delivery of the head by extension is the subocciput against the subpubic angle (inferior border of symphysis pubis).

A common vignette trap: in an occipito-posterior (OP) position, internal rotation of 135° (long rotation) brings the occiput anteriorly in ~90%; failure → persistent OP, delivery as "face-to-pubis" with extra perineal stretch.

Partograph & cervicograph

The partograph is a graphical record of labour progress used to detect abnormal labour early. WHO's modified partograph plots:

• Cervicograph: cervical dilatation (cm) vs time, with an alert line and a action line (4 h to the right of alert). Older WHO partograph started plotting at 4 cm.
• Descent of head (in fifths palpable abdominally).
• Fetal heart rate, liquor (membranes/colour), moulding, caput.
• Maternal pulse, BP, temperature, urine, contractions per 10 min, oxytocin/drugs.

High-yield: On the partograph, when the cervical dilatation curve crosses the alert line, the patient should be shifted/referred to a facility capable of caesarean; crossing the action line mandates active intervention/intervention decision.

Note: WHO's 2020 Labour Care Guide (LCG) replaces the older partograph in many programmes (uses the 5 cm active-phase threshold and individualised reference ranges rather than rigid alert/action lines), but for NEET PG the classic alert/action-line partograph remains examinable.

Management of normal labour (drug & active management)

First stage: monitor with partograph; allow oral fluids/light diet (low-risk), encourage mobilisation/upright positions, analgesia (epidural is the gold standard for labour analgesia). Avoid routine early amniotomy.

Second stage: support spontaneous pushing after full dilatation; controlled delivery of head; restrictive (not routine) episiotomy; deliver shoulders without excess traction (avoid shoulder dystocia mismanagement).

Third stage — Active Management of Third Stage of Labour (AMTSL) to prevent PPH:

1. Uterotonic within 1 minute of delivery — Oxytocin 10 IU IM is the drug of choice (WHO first-line).
2. Controlled cord traction (Brandt–Andrews) with counter-pressure on the uterus.
3. Uterine massage after placental delivery.

High-yield: Oxytocin 10 IU IM is the uterotonic of choice for AMTSL. Where oxytocin is unavailable, misoprostol 600 µg oral is the alternative. Carbetocin (heat-stable) is a WHO-endorsed option where cold chain is poor.

Signs of placental separation: (1) gush of blood, (2) lengthening of cord, (3) fundus becomes globular/firm and rises (Schroeder's sign), (4) suprapubic bulge. Mechanisms: Schultze (central, fetal surface first — commoner) vs Matthews-Duncan (marginal, maternal surface first, more bleeding).

Fourth stage: watch for PPH; ensure uterus contracted; inspect placenta for completeness and the genital tract for tears.

Complications / abnormal patterns to recognise

Pattern	Definition	Common cause
Prolonged latent phase	>20 h primi / >14 h multi	Unripe cervix, sedation
Protracted active phase	Dilatation <1.2 (primi)/1.5 (multi) cm/h	Inadequate powers, CPD, malposition
Arrest of dilatation	No change ≥4 h (adequate contractions), ≥6 cm	CPD, malposition
Arrest of descent	No descent in 2nd stage	CPD, malposition, inadequate effort
Precipitate labour	Total labour <3 h	Hyperstimulation, low resistance

Other complications of even "normal" labour: perineal tears, postpartum haemorrhage (commonest cause = uterine atony), retained placenta, cord prolapse, fetal distress, and shoulder dystocia.

Key differentials & related concepts

• False labour / Braxton-Hicks vs true labour (table above).
• Cephalopelvic disproportion (CPD): suspect with arrest patterns, high head at term in primi, excessive moulding (grade 3) + caput.
• Malposition (OP/OT) vs malpresentation (brow, face, breech): malposition = abnormal position of vertex; malpresentation = non-vertex presenting part. Distinguish because brow (mentovertical 14 cm) typically obstructs.
• Constriction ring (Bandl's ring pathological): sign of obstructed labour — the retraction ring rises and becomes visible/palpable suprapubically.

Recently asked / exam angle

• Engaging diameter of well-flexed vertex = suboccipitobregmatic 9.5 cm (perennial single-best-answer).
• Largest fetal AP diameter / brow presentation diameter = mentovertical 14 cm.
• Movement responsible / structure causing internal rotation = pelvic floor (levator ani); level = ischial spines.
• Only movement present throughout labour = descent.
• Station 0 corresponds to the presenting part at the ischial spines = engagement.
• Revised active-phase threshold = 6 cm; arrest of labour diagnosis criteria (≥6 cm, ROM, no change ≥4 h) — increasingly tested as ACOG guidelines update.
• Drug of choice in AMTSL = oxytocin 10 IU IM; misoprostol 600 µg as alternative.
• Fulcrum for head delivery = subocciput against inferior symphysis pubis (delivery by extension).
• Diagram-based: identify the cardinal movement from a labelled fetal-head illustration, or read the partograph alert/action line.
• Schultze vs Matthews-Duncan placental separation — central vs marginal.
• Ferguson reflex — cervical stretch → oxytocin release.

High-yield: Examiners love pairing "which cardinal movement" with "what causes it" — link flexion → pelvic floor & inlet resistance (gives SOB 9.5 cm) and internal rotation → levator ani slope at ischial spines.

Rapid revision

1. True labour = progressive cervical dilatation + descent with regular contractions; show = mucus plug expulsion.
2. Cardinal movements (OA): Engagement → Descent → Flexion → Internal rotation → Extension → Restitution → External rotation → Expulsion.
3. Descent is the only movement occurring throughout labour.
4. Well-flexed vertex engages with suboccipitobregmatic = 9.5 cm; internal rotation 45° at the ischial spines by the levator ani.
5. Head delivers by extension, pivoting at the subocciput against the subpubic symphysis.
6. Mentovertical 14 cm = largest diameter = brow presentation (usually obstructed).
7. Engagement = biparietal diameter through inlet = head ≤2/5 palpable abdominally = station 0.
8. First stage latent → active; revised active phase begins at 6 cm; arrest at ≥6 cm with ROM and no change ≥4 h.
9. Friedman rates: 1.2 cm/h primi, 1.5 cm/h multi; second-stage limits 3 h primi / 2 h multi (+1 h with epidural).
10. Partograph: cervix crossing the alert line → refer; crossing action line → intervene.
11. AMTSL = oxytocin 10 IU IM + controlled cord traction + uterine massage; misoprostol 600 µg if no oxytocin.
12. Placental separation: Schultze (central, commoner) vs Matthews-Duncan (marginal, more bleeding); commonest cause of PPH = uterine atony.