Induction of labour

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Chapter 38 Induction of labour

Michael Flynn

Assessing the cervix

The cervix remains closed because of its rigidity due to the collagen fibres that make up the bulk of cervical stroma. Cervical connective tissue consists mainly of collagen and a matrix of large proteoglycan molecules.

Cervical changes are due to:

• changes in proteoglycan
• collagen degradation
• increased vascularity
• accumulation of interstitial fluid

A uniform means of assessing the cervix is the Bishop’s score (see Table 38.1). The Bishop’s score assesses the favourability of the cervix for induction of labour by assigning points to each of the five cervical features and adding these points. With a low score (0–3), there is a high risk of a failed induction, resulting in caesarean section (>20%), compared with a score of 8 or more, where the failed induction rate is <3%. With a high score, the cervix is said to be ‘ripe’.

Table 38.1 The Bishop’s score

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Mechanism of labour

The exact mechanism of labour is still unknown, although it is thought that the production of fetal cortisol increases placental oestrogen and prostaglandin production, and sensitises the myometrium to oxytocin. Other factors such as progesterone, relaxin and prostacycline dominate early in pregnancy to inhibit contractility. Prostaglandin E2 and F2-alpha are synthesised by the decidua and amnion, and may sensitise the myometrium to oxytocin.

Induction of labour

Uterine rupture can occur with any agent that enhances uterine tone. The sensitivity of the myometrium to prostaglandin and oxytocin rises as gestation increases. However, the myometrium is relatively insensitive to oxytocin before term.

Cervical favourability (Bishop’s score) is an indicator of the myometrial sensitivity to oxytocin. Prostaglandin may augment the action of oxytocin and lead to uterine rupture.

Indications

• prolonged pregnancy (the Canadian Multicenter Post-Term Pregnancy Trial compared induction of labour and conservative management in uncomplicated pregnancies >41 weeks gestation: the results showed lower caesarean section rates in the induction-of-labour group, with both groups showing similar perinatal mortality and neonatal morbidity rates)
• hypertensive disorders of pregnancy
• premature rupture of membranes
• intrauterine fetal death
• diabetes
• intrauterine growth restriction
• antepartum haemorrhage
• isoimmunisation
• unstable lie
• maternal diseases

Contraindications

Table 38.2 lists contraindications for induction of labour.

Table 38.2 Contraindications for induction of labour

Absolute Relative
Absolute cephalopelvic disproportion Antepartum haemorrhage
Presumed fetal distress Grand multiparity
Placenta praevia Previous caesarean section
Vasa praevia Overdistended uterus
Abnormal presentation Face or breech presentation
Previous classical caesarean section History of rapid labour
Invasive carcinoma of cervix  
Cord presentation  

Techniques of induction

Surgical

‘Sweeping’ of the membranes

• Women who undergo daily sweeping of membranes for 3 consecutive days appear more likely to go into labour than controls.

Amniotomy

• This method of surgically rupturing the forewaters with an amnihook or other instrument is often used in combination with oxytocin.
• Risks include cord prolapse, intrauterine infection (especially with an increased induction-to-delivery interval) and an increased incidence of cardiotocograph abnormalities.

Medical

Oxytocin

• In vivo, oxytocin is synthesised in the paraventricular nucleus of the hypothalamus and is transported to the posterior pituitary gland. It is released as a free peptide in response to suckling/nipple stimulation, genital stimulation and stretching of the cervix. Increased sensitivity of the myometrium occurs with increasing gestational age.
• Although regimens vary, the required oxytocin dose to provide adequate uterine action is usually between 4 and 16 milliunits per minute. As the physiological dose is individualised, low doses are used initially with close monitoring of contractions, uterine relaxation and progress.
• Complications include water retention, hyponatraemia, uterine hyperstimulation and rupture, and neonatal hyperbilirubinaemia.
• Compared with amniotomy alone, the combined use of amniotomy and oxytocin infusion increases the likelihood of delivery within 24 hours, and lowers the risk of operative delivery and of postpartum haemorrhage.

Prostaglandin

The favourability of the cervix is the best available predictor of a successful induction of labour. At present, the most effective method of cervical ripening is the use of local prostaglandins. When comparing local prostaglandin induction with amniotomy/oxytocin induction, prostaglandin is associated with a decrease in length of labour, lower caesarean section rates (by reducing the number of failed inductions), and fewer Apgar scores (at 1 minute) below 4.

Types of prostaglandin agents

Prostaglandin E2 is used for cervical ripening for induction of labour. It is manufactured in a triactin-based gel with 1 or 2 mg dinoprostone in each unit dose of 3 g (2.5 mL). A slow-release delivery system vaginal insert is also available.

Actions of prostaglandins

These soften and efface the cervix by a combination of reducing the collagen concentration and changing the glycosaminoglycan composition and hydration.

Other effects

Prostaglandin E2 produces vasodilatation and 30% increased cardiac output. It relaxes bronchial and gastrointestinal smooth muscle.

Contraindications of prostaglandin E2

These include grand multiparity, rupture of membranes, high presenting part, past uterine surgery, cephalopelvic disproportion, abnormal cardiotocograph, malpresentation and unexplained vaginal bleeding.

Side effects of prostaglandins

These include uterine hyperstimulation (<1%), irritation of the vagina, nausea, vomiting, diarrhoea, pyrexia, broncho-constriction, hypertension, blurred vision, facial flush and vasovagal reaction.

Augmentation of labour

Aims

• accelerate progress of labour
• reduce operative vaginal delivery rates
• lower caesarean section rates
• lower the need for analgesia
• reduce the psychological impact associated with slow labour
• primary role of augmentation is to reduce the rate of dysfunctional labour in primigravid women, which may occur in up to 40% of cases

Methods

• Environment: the presence of a support person may reduce the length of labour.
• Ambulation: this reduces the need for oxytocic augmentation.
• Increase uterine contractility: amniotomy, oxytocin (50% of those diagnosed with slow labour will progress equally with or without oxytocin; 95% of primigravidas in labour with oxytocin augmentation will deliver within 12 hours).

Considerations in augmentation

The differential diagnosis of slow progress in labour includes:

• reduced uterine contractility
• increased resistance in soft tissues (relative disproportion, including malpresentation)
• absolute cephalopelvic disproportion

To differentiate between these mechanisms, adequate uterine contractility is required and oxytocics are suggested.

Further reading

Bishop E.H. Pelvic scoring for elective induction. Obstetrics and Gynecology. 1964;24:266.

Grant J.M. Sweeping the membranes in prolonged pregnancy. British Journal of Obstetrics and Gynaecology. 1993;100:889-890.

Hannah M., Hannah W.J., Hellmann J., et al. Induction of labor as compared with serial antenatal monitoring in post-term pregnancy: a randomized controlled trial. Canadian Multicenter Post-Term Pregnancy Trial Group. New England Journal of Medicine. 1992;326(24):1587-1592.

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