Normal labour: mechanism, stages and management

Overview

Normal labour is labour that:

• starts spontaneously between 37 and 42 weeks;
• involves a single fetus presenting by the vertex (cephalic);
• progresses at a normal rate without artificial intervention;
• ends in a spontaneous vaginal birth, with a third stage of 30 minutes or less under active management.

Progress depends on the "3 Ps": powers (uterine contractions and maternal pushing), passages (bony pelvis and soft tissues) and passenger (fetal size, lie, presentation, position and attitude).

The passages: maternal pelvis

• Gynaecoid pelvis: commonest and best for labour.
• Android: deep transverse arrest, failed rotation. Anthropoid: occipito-posterior position. Platypelloid: failure to engage.
• The levator ani forms a sloping gutter that helps the head flex and rotate.
• Station: −1 is 1 cm above the spines, +1 is 1 cm below. Instrumental delivery needs the head at or below the spines.

The passenger: fetal skull

• Anterior fontanelle (bregma): diamond shaped; four sutures meet (sagittal, frontal, two coronal).
• Posterior fontanelle: smaller, triangular; three sutures meet (sagittal and two lambdoid). It marks the occiput and so defines the position.
• Vertex: area between the two fontanelles and the parietal eminences.
• Moulding: overlap of skull bones (parietals slide over frontal and occipital bones) to reduce head diameters. Severe or early moulding suggests cephalopelvic disproportion or malposition.

Presenting diameters

The powers: physiology of labour

• Progesterone keeps the uterus quiet. Before labour, progesterone action falls and oestrogen rises relative to it.
• Prostaglandins (from decidua and membranes) and oxytocin raise intracellular calcium and trigger contractions; gap junctions multiply at term so the myometrium contracts as one unit.
• Ferguson reflex: pressure of the presenting part on the cervix increases oxytocin release from the maternal posterior pituitary.
• Retraction: upper segment fibres shorten with each contraction, so the upper segment thickens and the lower segment thins, pulling up the cervix (effacement) before it dilates.
• Cervical ripening: collagen breaks down and water content rises, so the cervix softens.
• Contractions: every 2-4 minutes, lasting 30-60 seconds; advanced labour 4-5 in 10 minutes.

Diagnosis of labour

Labour = regular painful contractions with progressive effacement and dilatation of the cervix.

Stages of labour

• Average total labour: about 8 hours in a first labour and 5 hours in later ones.
• Effacement then dilatation happen one after the other in nulliparas but together in multiparas.
• Prolonged labour: more than 12 hours (nullipara) or 8 hours (multipara) in Ten Teachers' working definition.
• Precipitate labour: birth within 3 hours of the start of regular contractions.
• Whatever the epidural, the baby should be born within 4 hours of full dilatation.

Mechanism of labour (vertex, left occipito-anterior)

The cardinal movements in order:

1. Engagement: the widest part (biparietal diameter) passes the inlet, usually transverse. Engaged = 2/5 or less of the head palpable abdominally. In nulliparas this usually happens by 37 weeks; in multiparas often only in labour.
2. Descent: by contractions, then helped by maternal pushing (Valsalva).
3. Flexion: the head flexes as it meets the narrower cavity, presenting the 9.5 cm diameter.
4. Internal rotation: the occiput meets the sloping levator ani gutter and rotates forwards so the sagittal suture lies in the AP diameter of the outlet. Happens at the level of the ischial spines.
5. Extension: the occiput escapes under the symphysis (crowning); the sinciput, face and chin sweep over the perineum.
6. Restitution: the head turns one-eighth back to line up with the shoulders.
7. External rotation: a further one-eighth turn as the shoulders rotate into the AP diameter of the outlet.
8. Delivery of shoulders and body: anterior shoulder under the symphysis first (gentle downward traction), then posterior shoulder (upward), then the body.

Management of the first stage

On admission

• History: contractions, membranes (colour and amount of liquor), bleeding, fetal movements, previous births and caesarean scar, risk factors.
• Examination: pulse, BP, temperature, urine; abdomen for lie, presentation and fifths palpable; contractions felt for at least 10 minutes.
• Vaginal examination: cervical dilatation, effacement, consistency, position; station relative to the spines; position (find the posterior fontanelle); caput and moulding; membranes and liquor.

Warning signs: a high head (5/5) at term in a nullipara, meconium-stained or blood-stained liquor, scanty liquor, malposition.

Observations in established labour

Continuous CTG is advised for meconium, abnormal heart rate on auscultation, maternal fever, fresh bleeding, oxytocin use, or maternal request (see the chapter on fetal monitoring and CTG).

Supportive care

• One-to-one midwifery care (the one part of the old "active management of labour" package with proven benefit).
• Birth partner, mobility and upright positions; avoid lying flat.
• Oral fluids and light diet if low risk; IV fluids if ketotic.
• Latent phase: manage away from the labour ward, simple analgesia, reassurance; avoid ARM and oxytocin, which raise the chance of later problems.
• Pain relief: simple analgesia, nitrous oxide and oxygen (Entonox), opioids (pethidine, diamorphine), epidural (the most effective).

The partogram

A single chart of labour: cervical dilatation and descent (fifths and station) against time, plus contractions, fetal heart, liquor, moulding, drugs and maternal observations.

• Start it in the active phase (not during the latent phase).
• The expected line runs at 1 cm per hour; the action line is drawn 4 hours to the right of it.
• Crossing the action line, or dilatation under 2 cm in 4 hours, means slow progress.

Management of the second stage

• Confirm full dilatation by VE if the head is not visible.
• Let her push with her own urge, in any comfortable position (left lateral, squatting, all fours); avoid supine.
• With an epidural, delay pushing for 1-2 hours (passive second stage).
• Crowning: the head stops slipping back between contractions; ask her to pant rather than push so the head delivers slowly.
• Episiotomy: a mediolateral cut (usually to the right) from the fourchette; used for instrumental birth or fetal compromise, not routinely, and it does not prevent anal sphincter tears.
• After the head: feel for cord round the neck; then deliver shoulders with traction on the head, never pulling on the body by the head if the baby is large (see shoulder dystocia in obstetric emergencies).

Perineal tears

Third- and fourth-degree tears are obstetric anal sphincter injuries (OASI) and are repaired in theatre by an experienced operator. If unsure, classify upwards.

Immediate care of the baby

• Delay cord clamping (cord pulsation adds about 80 mL of blood and reduces later anaemia).
• Apgar at 1 and 5 minutes; dry, keep warm, skin-to-skin; start breastfeeding within the first hour; vitamin K.

Management of the third stage

How the placenta separates

The uterus contracts and retracts, the placental site shrinks, and a cleavage plane forms in the decidua basalis.

Signs of placental separation:

• The cord visibly lengthens.
• A gush of blood from the vagina.
• The fundus rises (above the umbilicus) and becomes firm, globular and mobile.

Active versus physiological management

• If controlled cord traction fails and there is no bleeding, try again after 10 minutes; if still undelivered, the placenta is retained → manual removal under regional or general anaesthesia.
• Pulling the cord without guarding the uterus or before separation risks uterine inversion.
• Examine the placenta for missing cotyledons or a succenturiate lobe (risk of PPH and infection), then examine the perineum and vagina for tears.

Quick revision

• Normal labour: spontaneous, 37-42 weeks, singleton vertex, spontaneous vaginal birth.
• Stage 1 to 10 cm; stage 2 to the baby; stage 3 to the placenta.
• Active phase from about 4 cm; slow if under 2 cm in 4 hours; action line 4 hours right of the 1 cm/hour line.
• Active second stage: 2 hours nullipara, 1 hour multipara; birth within 4 hours of full dilatation.
• Smallest diameter: suboccipitobregmatic 9.5 cm; brow (mentovertical 13 cm) cannot usually deliver.
• Cardinal movements: engagement and descent, flexion, internal rotation, then extension; after birth of the head, restitution and external rotation; finally expulsion.
• Separation signs: cord lengthens, gush of blood, fundus rises and becomes globular.
• Active third stage: oxytocin 10 IU IM + cord clamping + controlled cord traction; PPH 15% → 5%.

Written by The House Job from standard references (Obstetrics by Ten Teachers).

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