Induction and augmentation of labour

Overview

Induction of labour is the planned starting of labour before it begins by itself; it precedes about a fifth to a quarter of deliveries in developed settings. Augmentation means strengthening contractions once labour has already begun. The principle for induction is simple: it is done when continuing the pregnancy is more dangerous for mother or baby than ending it, and only where success is reasonably likely and the risks of the process itself are acceptable.

Indications

• Prolonged pregnancy (the commonest). Beyond 42 weeks stillbirth is commoner, as are fetal compromise during labour, aspiration of meconium and mechanical trouble at birth, so induction is usually recommended between 41 and 42 completed weeks. Induction for this reason does not raise the caesarean section rate. Women who decline are offered closer fetal monitoring.
• Prelabour rupture of membranes. At term, induction about 24 hours after membranes rupture reduces chorioamnionitis, endometritis and neonatal unit admission. Before 34 weeks another indication is needed; between 34 and 37 weeks the decision is individual.
• Pre-eclampsia and other hypertensive disease (at term; very preterm or rapidly deteriorating disease may need caesarean delivery).
• Fetal growth restriction.
• Diabetes, twin pregnancy beyond 38 weeks and intrahepatic cholestasis of pregnancy, commonly around 38 weeks.
• Suspected macrosomia above the 90th centile without diabetes (remember the 10-20% error of ultrasound weight estimation).
• Deteriorating maternal illness, unexplained antepartum haemorrhage, red cell isoimmunisation.
• "Social" reasons: discouraged, needing careful counselling, and more acceptable only with higher parity and a favourable cervix.

Contraindications

The Bishop score

As spontaneous labour approaches, the cervix softens, shortens, moves forward, effaces and begins to dilate. The modified Bishop score grades these changes.

Methods, in order of use

Membrane sweep

A gloved finger is passed through the cervix and swept around the inner rim, separating the chorionic membrane from the decidua and releasing natural prostaglandins. It is safe, can be repeated, and reduces the need for formal induction. It is offered at term, usually from about 40 weeks, requires the cervix to be beginning to dilate and efface, and placenta praevia must be excluded first. It is uncomfortable. Treat it as an adjunct, not a method on its own.

Vaginal prostaglandin E2

• Dinoprostone (PGE2) placed in the posterior vaginal fornix as a tablet or gel, or as a controlled-release pessary left for up to 24 hours.
• Two doses of the tablet or gel are often needed, given at least 6 hours apart.
• It ripens the cervix: proteoglycan content rises and the physical properties of cervical collagen alter, as well as stimulating contractions.
• A prostaglandin may still be given where the cervix is already favourable.
• Labour may follow prostaglandin alone, but artificial rupture of membranes and oxytocin are often needed as well, especially in a first pregnancy.

Artificial rupture of membranes (amniotomy)

The traditional method, now used when the cervix is favourable. It allows oxytocin to be started and lets the colour of the liquor be seen.

Oxytocin infusion

• A nine-amino-acid peptide from the posterior pituitary, with a short half-life (about 5 minutes), so it is given as a dilute intravenous infusion by pump, never as a bolus for induction.
• It works at G protein-coupled receptors, using the phosphoinositide and calcium messenger system, to contract myometrium, and also releases prostaglandins. Small doses raise the frequency and force of contractions; large doses give sustained contraction.
• The membranes must be ruptured first, spontaneously or artificially.
• Response varies greatly between women, so a strict protocol is used: start at a low rate and increase by set increments every 30 minutes until 3 to 5 contractions every 10 minutes are achieved.
• Continuous electronic fetal monitoring is needed.
• Myometrial sensitivity to oxytocin rises sharply at term because oxytocin receptor numbers increase, not because oxytocin production rises.

Mifepristone with misoprostol

Mifepristone (an antiprogesterone) with misoprostol (a PGE1 analogue) will induce labour, but complication rates appear higher; in UK practice the combination is kept for induction after intrauterine fetal death. Misoprostol is cheap and heat-stable, so local practice in low-resource settings may differ: follow the current national guideline.

Augmentation of labour

• Suspect delay, re-examine at 2 hours rather than 4; if delay is confirmed offer artificial rupture of membranes, and if progress is still poor after a further 2 hours seek obstetric advice about oxytocin.
• Start slowly and increase every 30 minutes to protocol, with continuous monitoring, fluids and support. Offer an epidural before oxytocin is started.
• If there is no progress after 4-6 hours of augmentation, caesarean section is usually advised.
• Augmentation is contraindicated if there is concern about fetal condition.
• In a multiparous woman, poor progress is more often malposition, malpresentation or obstruction: an obstetrician must assess her. In a multiparous woman with suspected cephalopelvic disproportion, oxytocin is never given, because excessive contractions in truly obstructed labour can rupture the uterus, a complication that is very rare in a first labour. Careful oxytocin may be given to a primigravida with mild to moderate disproportion provided the trace is normal.

Complications

Failed induction

Induction has failed when the maximum number of prostaglandin doses has been given and amniotomy remains impossible, or when, after amniotomy, regular contractions on oxytocin for 6-8 hours leave the cervix uneffaced and under 3 cm dilated. Options are a rest period and a further attempt later, or caesarean section. Waiting is acceptable only where neither mother nor fetus is threatened, as after a failed social induction; failure in pre-eclampsia or growth restriction usually means caesarean delivery.

Related uterotonics to recognise

Worked example: titrating the infusion

Quick revision

• Induction is offered when continuing the pregnancy is riskier than delivery.
• Commonest reason is prolonged pregnancy, at 41-42 weeks; it does not raise the caesarean rate.
• Bishop score grades dilatation, length, consistency, position and station.
• Sequence: sweep, vaginal PGE2, amniotomy, oxytocin infusion.
• Oxytocin: dilute infusion, short half-life, increase every 30 minutes to 3-5 contractions per 10 minutes.
• Hyperstimulation is over 5 contractions in 10 minutes; stop oxytocin, give terbutaline.
• Myometrial oxytocin sensitivity rises at term because receptor numbers rise.
• High oxytocin doses act on vasopressin receptors and cause water intoxication with hyponatraemia.
• Failed induction: amniotomy still impossible, or under 3 cm after 6-8 hours of oxytocin.
• Never use oxytocin in a multiparous woman with suspected disproportion.
• Mifepristone with misoprostol is used after intrauterine fetal death.

Written by The House Job from standard references (Obstetrics by Ten Teachers; Basic and Clinical Pharmacology by Katzung).

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