Cephalopelvic disproportion

Cephalopelvic disproportion (CPD) is failure of the fetal head to descend through the pelvis despite the presence of efficient uterine contractions and moulding of the fetal head. It occurs when the presenting diameter of the fetal head is larger than the diameters of the maternal pelvis through which it has to pass. Malpresentations, malpositions, pelvic tumours and fetal abnormalities which prevent the head descending through the pelvis are viewed as obstruction rather than as causes of cephalopelvic disproportion.

The head is the largest part of the fetus and once it has passed through the brim of the pelvis (see Ch. 24) the rest of the fetus should pass through without difficulty (but always remain mindful of shoulder dystocia). The probability is that the cavity and outlet are also of adequate dimensions to accommodate the passage of a normal fetus. However, in practice, there can be cephalopelvic disproportion at the cavity or outlet of the pelvis too. The reader should bear in mind the different types of pelves (see Ch. 24) and how these may influence the way the fetal head negotiates its passage through the bony canal.

During the last 2 or 3 weeks of pregnancy it will sometimes be found that the head is not engaged. The most common reason for non-engagement is an occipitoposterior position of the fetal head. The deflexed head results in a larger presenting diameter (the occipitofrontal diameter of which is 11.5 cm). In labour, however, the head usually flexes, and descends into the pelvis.

Diagnosis

The possibility of disproportion should be considered if there is a history of:

• medical conditions such as rickets or osteomalacia which could adversely affect the size and shape of the pelvis

• spinal deformities such as scoliosis

• pelvic fractures or injuries which may have altered the normal shape and dimensions of the pelvis

• obstetric complications such as previous prolonged labour, difficult birth, caesarean section or perinatal death.

The possibility of cephalopelvic disproportion might also be considered from the collective assessment of:

• the woman’s stature – particularly if less than 150 cm

• size of fetus if unduly large

• the woman’s body mass index (BMI).

Regarding the above points, Hofmeyr (2004) suggests uncertainty as to the use of maternal height as a predictor for CPD. Whilst it is difficult to assess the size of the fetus accurately by palpation, with experience it is possible to determine whether the fetus is unduly large for the duration of the pregnancy. Despite improvements in ultrasound technology, and more accurate estimation of fetal size, Hofmeyr (2004) questions the value of this practice. Technology is no substitute for the expert midwifery skills required to obtain an accurate medical and obstetric history, or for midwives developing the clinical expertise to assess fetal and pelvic size and interpret the idiosyncratic combinations of all the above findings and take appropriate action. Finally, of note, is an increasing body of evidence pointing to the association of raised BMI and cephalopelvic disproportion (Roman et al 2008).

The possibility is that the presence of disproportion is much greater in a primigravida than in a multigravida who has had a history of previous normal deliveries, though it can never be ruled out, since the size of the fetus may have been smaller in previous pregnancies. Worthy of mention here is that disproportion resulting from excessive fetal size may be controlled by good management of maternal diabetes, a predisposing factor in macrosomia.

Reflective activity 65.1

Consider what midwifery skills and strategies could be employed that may enhance/encourage flexion and descent of the fetal head through the pelvis.

X-ray pelvimetry

In recent years, the benefit of X-ray and magnetic resonance imaging (MRI) pelvimetry has been questioned (Zaretsky et al 2005). Ikhena et al (1999) point out that there are no agreed measurements considered satisfactory to allow vaginal birth, and, in addition, pelvimetry may serve to increase the caesarean section rate. Furthermore, Pattinson (1998) concludes that even in situations of anticipated cephalopelvic disproportion, women do better if allowed to labour. The evaluation of the progress of labour is considered a far more accurate indicator of cephalopelvic disproportion (Chhabra et al 2000, Impey & O’Herlihy 1998). Hence, there are only rare situations where pelvimetry is indicated (for example, history of fractured pelvis).

From these examinations and assessments, the mother will fall into one of three categories:

1 No disproportion is evident and spontaneous labour and vaginal birth can be awaited.

2 There is a slight degree of disproportion which may be overcome successfully in labour. Providing there are no other complications, the woman is admitted to hospital for a trial of labour.

3 There is evidence of disproportion, either because the pelvis is small or abnormal in shape or because the fetus is unusually large. Vaginal birth is out of the question and caesarean section is necessary.

Dangers

If cephalopelvic disproportion is not detected, it will lead to obstructed labour, which may result in a ruptured uterus and in maternal and fetal death.

Trial of labour

A trial of labour is an ordinary labour conducted in hospital when there is a minor degree of cephalopelvic disproportion. The aim is to ascertain if the contractions of labour will flex and mould the fetal head sufficiently to make it engage and descend through the pelvis. If the head engages, labour is likely to continue normally. The purpose is to give maximum opportunity for the woman to benefit from a vaginal birth within the parameters of fetal and maternal safety. A successful trial of labour prevents unnecessary operative intervention and the associated risks of increased morbidity and mortality, including psychological problems. It also influences the management of future labours, in that if the woman delivers safely vaginally, this is likely to be the pattern in the future.

It is considered that all primigravidae with a non-engaged head are undergoing a trial of labour. With careful selection in the antenatal period and using an upright position in labour, or active management of labour to ensure effective uterine action, most will experience a normal birth. A hands–knees posture may be of benefit to encourage anterior rotation of the lateral or posterior head positions in the pelvis (Lewis et al 2002).

Conditions necessary in labour when a minor degree of cephalopelvic disproportion is suspected

The presentation must be cephalic (including occipitoposterior position). The midwife should be conscientious in excluding a brow presentation, which would contraindicate a trial of labour. Similarly, a trial of labour is not carried out in breech presentations. Further to this, there should be no major degree of cephalopelvic disproportion. The woman should be young and healthy with an uncomplicated medical and obstetric history. There should be no complications of pregnancy such as hypertension or antepartum haemorrhage and the pregnancy must not be postmature, otherwise the fetal head may not mould satisfactorily. There is strong evidence to suggest that it is safe to conduct a trial of labour with a transverse lower segment uterine scar, providing there is careful management and monitoring in labour (Brill & Windrim 2003).

Management

Labour should take place in an obstetric unit where there are both the facilities and the personnel available for electronic monitoring and any interventions that may be required to achieve a safe outcome in labour for both mother and baby. Good preparation of the woman and her partner before the onset of labour is essential to gain their understanding of the situation and their cooperation in labour. When faced with obstetric emergencies and the need to minimize morbidity and mortality, available options/choices may be diminished. However, women and their partners can still experience a sense of control through the sharing of information and in the decision-making process.

During labour the mother is encouraged to be ambulant because an upright position promotes flexion and descent of the head, cervical dilatation and the maintenance of contractions (Lewis et al 2002). Continuous electronic monitoring of the fetal heart and uterine contractions is recommended.

Progress is assessed by:

• the efficiency of uterine contractions

• abdominal palpation to determine progressive descent of the fetal head

• vaginal examination to note descent and position of the head, effacement of the cervix and dilatation of the os uteri.

All observations and findings of examinations are plotted on a partogram (see Ch. 36). This facilitates rapid recognition of any delay in cervical dilatation and descent of the head and indicates when action should be taken to aid progress. Once labour is established, cervical dilatation of less than 1 cm per hour over a 2-hour period is often regarded as delayed progress (Enkin et al 2000), though there may be local specific protocols for a trial of labour.

If progress is found to be slow due to inefficient uterine action, active management of labour may be undertaken, after careful assessment by an obstetrician. Oxytocic drugs can be used to overcome active phase arrest that is unrelated to cephalopelvic disproportion (Foley et al 2004). Otherwise a diagnosis of cephalopelvic disproportion may be made in cases when the real problem is inefficient uterine action. The use of oxytocin in a trial of labour may be viewed as controversial because of the risk of uterine rupture. However, there is support for its use, provided that continuous electronic cardiotocography is carried out and facilities for the management of any emergencies are readily available (ACOG 2004). Having said that, an increase in the incidence of ruptured uterus is associated with the overuse of Syntocinon. It should be stressed that active management would never be undertaken if more than a minor degree of cephalopelvic disproportion was suspected – emphasizing selective and judicious use of oxytocin.

Conditions necessary for the use of oxytocin in a trial of labour can be summarized thus:

• expert midwifery care to monitor uterine activity both by abdominal palpation and by skilled interpretation of the cardiotocograph trace

• continuous electronic cardiotocography

• close surveillance of fetal and maternal wellbeing

• avoidance of hyperstimulation of uterine activity by oxytocin and, in the absence of significant progress, a limited period of use.

If uterine hyperstimulation or fetal distress occurs, the oxytocin is immediately stopped and the obstetrician informed. If progress in labour does not improve with the use of oxytocin for a limited period, caesarean section is indicated.

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