INDUCTION OF LABOUR

Induction of labour may be required to ‘rescue’ the fetus from a potentially hazardous intra-uterine environment in late pregnancy for a variety of reasons, or because continuation of the pregnancy is dangerous to the expectant mother. Indications for induction of labour are listed in Table 24.1.

Table 24.1 Indications for induction of labour: Australia

The method adopted depends on:

Fig. 24.1 The relationship between the condition of the cervix and success rate of surgical induction.

The highest rate of success, (i.e. that the induction is followed by a vaginal birth within 24 hours) occurs in a woman whose cervix is favourable and whose Bishop score is 5 or more (Table 24.2).

Table 24.2 A method of cervical ‘scoring’ (Bishop score)

If the chances of success are evaluated as low, the doctor may recommend caesarean section.

Techniques of inducing labour

Labour may be induced by drugs, by the surgical technique of amniotomy, which is also known as artificial rupture of the membranes (ARM), or by mechanical stimulation of the cervix.

Induction of labour using drugs

Two agents are available: prostaglandins and oxytocin.

Prostaglandins

Three prostaglandins with different properties are used.

• PGE₁ gemeprost is available in pessary form or as an intracervical injection system. It is used in the second trimester of pregnancy for the termination of pregnancy and for the induction of labour in cases of fetal death in utero. It should be avoided in the third trimester because of the risk of uterine hypertonicity. Misoprostol is a synthetic PGE₁ analogue which compares favourably with PGE₁ and PGE₂ in its efficacy in inducing labour. It can be administered by placing a tablet (25 μg) in the posterior vaginal fornix every 4 hours. Oral administration appears to be less effective and has been reported to be associated with higher rates of uterine hyperstimulation.

• PGE₂ is available as a gel or in pessary form and may be introduced intravaginally or intracervically. The gel appears to be more effective and is associated with a lower risk of producing uterine hypertonicity. PGE₂ softens the cervical connective tissue and relaxes cervical muscle fibres, ‘ripening’ the cervix. PGE₂ may stimulate the production of PGF_2α, which will induce uterine contractions. PGE₂ produces cervical ripening when the cervical (Bishop) score is less than 5, and therefore amniotomy is contraindicated. It is available in a prefilled syringe in a dose of 1–2 mg, which is introduced into the posterior vaginal fornix. The patient’s pulse rate, fetal heart rate and uterine activity are monitored for 30 minutes, after which time she may walk about if she wishes, provided the pregnancy is not at high risk. If labour is not established within 6 hours the woman is reassessed. If the cervix is now favourable ARM is performed, otherwise a further dose of PGE₂ is given.

• A slow-release pessary (Cervidil). It releases a mean of 4 mg over 12 hours and can easily be removed by its tape if necessary.

Oxytocin (Syntocinon)

Oxytocin should always be administered by intravenous infusion, preferably using an infusion pump. If it is used alone, 50% of women will be in labour within 12 hours. Induction is more effective if preceded by PGE₂ vaginal pessaries or following amniotomy.

The infusion rate is increased by 5 mU/min every 30 minutes until contractions lasting longer than 60 seconds recur at 3–5-minute intervals. The maximum rate used is 60 mU/min. Because of the risk of water intoxication, the quantity of fluid infused should not exceed 1500 mL in 10 hours.

Surgical induction of labour (amniotomy)

Surgical induction of labour (amniotomy) is more effective if the cervix is favourable (Bishop score 5+). The patient is placed in the dorsal or lithotomy position and the vagina swabbed with antiseptic. Two of the doctor’s fingers are inserted to reach the cervix and, if practicable, one is inserted through the cervix to ‘sweep’ the membranes. An amnihook, or a Kocher forceps, is introduced along the intravaginal fingers and the membranes below the fetal head (the forewaters) are broken with the instrument.

The problems following amniotomy are:

• Prolapse of the umbilical cord (0.5% of cases)

• Infection, if the induction–delivery interval is >24 hours.

A suggested schema (flow chart) for the induction of labour is shown in Figure 24.2.

Fig. 24.2 Induction of labour flow chart.

As can be seen from the flow chart, if the cervix is ‘unfavourable’ (i.e. a low Bishop score), the drugs mentioned may be used to ripen the cervix so that surgical induction, if needed, will be more effective. In the flow chart PGE₂ is featured, but the other prostaglandins listed above are equally effective.

Mechanical methods

A single (e.g. Foley) or double (Atad) balloon catheter is also an effective method of inducing labour. The catheter is passed aseptically through the cervix and the balloon(s) are inflated with sterile saline. The woman is then free to ambulate and is assessed 6 or so hours later, unless she goes into labour or the membranes rupture. Other devices, such as the hydrophilic laminaria tent have been used. They swell by absorbing fluid and so slowly dilate the cervix.

The principal benefits of balloon catheters are the induction of labour in women who have previously had a caesarean section; this avoids the potential hyperstimulation that may occur with prostaglandins, or the theoretical softening of the collagen fibres in the uterine scar.

AUGMENTATION OF LABOUR

In cases where the quality of the uterine contractions is poor (see p. 174), their strength may be augmented by performing ARM and, if necessary, by setting up an incremental oxytocic infusion, or both.

INSTRUMENTAL DELIVERY

In the second stage of labour situations may arise wherein it becomes necessary to deliver the baby. These are:

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