Such indications are:
Hypertensive disorders.
Prolonged pregnancy.
Compromised fetus, e.g. growth restriction.
Maternal diabetes.
Rhesus sensitisation.
Hypertensive disease and prolonged pregnancy have long been the largest groups. Induction in such cases has often been carried out on epidemiological data as opposed to established risk in an individual case. Modern methods of fetal assessment aim to establish the risk in an individual and thus avoid needless intervention.
Other indications for induction are:
Fetal abnormality or death — the main reason for intervention is to alleviate distress in the mother.
Social — induction may be requested by a mother for a variety of social or domestic reasons. The obstetrician may reasonably agree to such requests if the findings are favourable for delivery and if there are no features which would make intervention unusually hazardous.
The effectiveness of modern methods of induction may tempt the obstetrician to be over-enthusiastic in their use. Any intervention should carry the implication of delivery by whatever means necessary and must therefore be justifiable.
METHODS OF INDUCTION
As labour approaches, the cervix normally shows changes known as ‘ripening’ so that it becomes ‘inducible’ and is then called a ‘favourable’ cervix. The condition of the cervix is the most important factor in successful induction and, where ripening has not occurred, there is a greater chance of a long labour, fetal hypoxia and operative delivery.
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The Bishop score
This is an accepted method of recording the degree of ripeness before labour (cf. the Apgar score applied to the newborn baby). It takes account of the length, dilatation and consistency of the cervix and the level of the fetal head. A score of 9 or higher is favourable.
| 0 | 1 | 2 | 3 | ||
|---|---|---|---|---|---|
| Dilatation (cm) | <2 | 2–4 | >4 | ||
| Length (cm) | >2 | 1–2 | <1 | ||
| Consistency | Firm | Average | Soft | ||
| Position | Post. | Mid Anterior | |||
| Level | 0–3 | 0–2 | 0–1:0 | 0+ | |
| Total |
(a). RIPENING THE CERVIX
The collagen fibres of which the cervix is composed can be much softened in consistency by the local application of prostaglandin. A vaginal tablet or gel containing dinoprostone (Prostin E2) may be inserted into the posterior fornix to soften and efface the cervix. This will permit amniotomy and may even result in the initiation of labour. Increasingly the cheaper alternative of misoprostol given intravaginally is being used for ripening the cervix and also for induction of labour.
Physical methods such as the introduction of a Foley catheter through the cervical os appear to stimulate local production of prostaglandins and may also ripen the cervix.
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(b). AMNIOTOMY (Artificial Rupture of Membranes)
This is done to initiate labour (surgical induction) or, during labour, to try to accelerate the process, or to allow a fetal scalp electrode to be applied or to permit estimation of the fetal pH. Amniotomy appears to release a local secretion of endogenous prostaglandins.
Amniotomy, using a Hollister Amnihook or other device, may be used to rupture the membranes overlying the presenting part. Care must be taken not to damage the fetal tissues. The operation may be done blindly by passing the instrument along the fingers or by direct vision using a speculum.
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The procedure is carried out using an aseptic technique and sometimes sedation or even epidural anaesthesia may be required to permit adequate examination. The colour and quantity of the liquor removed should be noted. Prolapse of the umbilical cord should be excluded at the beginning and end of the procedure.
Complications of Amniotomy
Failure to induce effective contractions
Labour may not become established after amniotomy alone and it is usual to stimulate the uterus further by intravenous oxytocin after an interval of 3 hours or so if contractions are inadequate.
Placental separation (Abruption)
This may be caused by the sudden reduction in the volume of liquor where there has been polyhydramnios.
Bleeding
This is not uncommon. The usual source is maternal blood from an element of forced dilatation of the cervix by the examining fingers. Occasionally it may come from fetal vessels running in the membranes (velamentous insertion of the cord). The best method of identifying the source of blood is by Kleihauer’s test, a laboratory procedure, by which a blood slide is so stained as to show the fetal cells standing out in a field of ‘ghost’ maternal cells (see Chapter 8).
Prolapse of the cord
This will only happen with an ill-fitting presenting part. Cord prolapse, occult or frank, should give warning signs on the fetal heart rate monitor.
Pulmonary embolism of amniotic fluid
This rare condition presents as severe shock of rapid onset, with intense dyspnoea and often bleeding. It is associated with amniotomy and strong uterine contractions, and must be distinguished from eclampsia, abruption, ruptured uterus, and acid aspiration. Treatment must include positive pressure ventilation, and correction of the inevitable coagulation defect. Postmortem examination of the maternal lungs will show fetal cells and lanugo.
(c). INTRAVENOUS OXYTOCIN
Synthetic oxytocin by continuous intravenous infusion is commonly used after amniotomy to stimulate uterine contraction. It is also used occasionally with intact membranes, e.g. to help stabilise the fetus with a variable lie prior to amniotomy. In this circumstance care should be taken to prevent excessive uterine action which can cause amniotic fluid embolism. Like amniotomy, intravenous oxytocin is also used to augment or accelerate labour.
Synthetic oxytocin is a powerful drug and sometimes unpredictable, as uterine sensitivity can show a wide variation. It must be administered with great care by the doctor or midwife who should be present throughout.
Effect on uterine activity
This varies with time and the progress of labour. Since too little oxytocin is useless and too much may cause fetal hypoxia or uterine rupture, it is necessary to adjust the dosage to the individual patient’s response.
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The best method of administration is by a suitable semi-automated infusion system incorporating an accurate drop counter. A solution of 2 units of Syntocinon in 500ml of Hartmann’s solution is used, beginning at a dose of 2.66mU/minute. This is increased every 15 minutes until satisfactory contractions are established.
Complications of oxytocin
Poor uterine action
This may occur where amniotomy has been carried out in spite of an unfavourable cervix. Ripening of the cervix with prostaglandin should be used first in these circumstances. Sometimes, in spite of apparently satisfactory uterine action, little dilatation of the cervix occurs and labour has to be terminated by caesarean section. This is due to incoordinate uterine action resulting in dysfunctional labour.
Abnormal fetal heart rate patterns
Prolonged or excessive oxytocin administration can cause fetal hypoxia by overstimulation of the uterus. Continuous fetal heart rate monitoring is required for all patients undergoing oxytocin stimulation.
Hyperstimulation
Overdosage can cause excessive, painful contractions and even a prolonged spasm (tetanic contraction). If hyperstimulation becomes evident, the infusion should be stopped to allow the uterus to relax.
An intra-uterine pressure transducer may be used in women who are difficult to assess, e.g. the obese.
Rupture of the uterus
The possibility of rupture must be borne in mind when using oxytocin. It is unlikely in a primigravida but has been reported. It is more to be expected in the parous woman or in the patient who has had a previous caesarean section or hysterotomy. The use of an intra-uterine pressure transducer may be advisable in such patients. Epidural anaesthesia does not mask the pain of uterine rupture but it should be used with caution.
Water intoxication
This may result from the prolonged administration of high doses of oxytocin in large volumes of electrolyte-free fluid. This should not be an issue in labour using normal dosage of oxytocin in an agent such as Hartmann’s solution.
ACCELERATION OF LABOUR
The progress of spontaneous labour can be speeded up by amniotomy and oxytocin infusion. By using these techniques most women can be delivered within 12 hours. Prolonged labour is thus avoided together with its possible accompaniment of maternal exhaustion, fetal distress and intra-uterine infection. Such interventions should not, however, be automatic and their indiscriminate application has aroused hostility in some mothers. If acceleration is considered desirable the reason for this should be explained and discussed with the mother.
FAILURE TO PROGRESS IN LABOUR
Failure of the cervix to dilate and for the presenting part to descend is a common event but it is important to appreciate that it is a clinical observation and not a diagnosis.
CAUSES OF FAILURE TO PROGRESS IN LABOUR
1. Incorrect diagnosis. Patient not in labour.
2. Dysfunctional uterine activity. Common in primigravidae, rare in multiparae. Associated with occipito-posterior malposition.
3. Malposition/malpresentation.
4. Cephalopelvic disproportion.
5. Rare causes, e.g. cervical stenosis from previous cervical surgery or pelvic tumour such as fibroid or ovarian cyst.
Dysfunctional Labour
Dysfunctional labour occurs when the cervix does not dilate despite the presence of uterine contractions. The cervix should dilate at a rate of between 1 and 2 centimetres per hour in the active phase. Progress is more rapid in parous women and dysfunctional labour is much commoner in primigravidae.
Treatment consists of escalating doses of oxytocin. This should only be used in parous women when there is no evidence of malpresentation, and even then only with caution. The dose of oxytocin should be titrated against the quality and frequency of uterine contractions.
Malposition/Malpresentation
Malposition means incorrect positioning of the vertex. This includes occipito-posterior (OP) positions and deflection of the head short of brow presentation.
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Malpresentation means the presence of any presenting part other than the vertex — face, brow, breech, shoulder, compound presentation.
MALPOSITION/MALPRESENTATION
Dangers
1. Ill-fitting presenting part. The forewaters are not protected from the forces of uterine contractions, and are forced through an incompletely dilated cervix.
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2. Membranes rupture early and the cord may prolapse past the presenting part.
3. Contractions may be irregular and poorly sustained. If moulding occurs, when the skull bones overlap, the presenting part may become a better fit and the labour will perhaps progress more normally; otherwise, dilatation of the cervix is likely to cease temporarily after the forewaters have ruptured.
4. In parous women labour may proceed quickly in spite of an ill-fitting presenting part.
With a malpresentation such as brow or shoulder there is a danger of obstructed labour and uterine rupture if unrecognised.
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