Chapter Forty-Four. Cephalopelvic disproportion, obstructed labour and other obstetric emergencies
Introduction
First, the birth canal and the pathway taken by the fetus are complex as the human pelvis is adapted to a bipedal posture (Morgan 1994). Compared to other primates such as the chimpanzee:
• The anteroposterior diameter is reduced at the brim, cavity and outlet.
• There is widening of the transverse diameters.
• The sacral promontory protrudes into the pelvic inlet.
• The sacrum makes an angle with the lumbar spine—the lumbosacral angle.
• There is inward protrusion of the ischial spines in order to support the strong pelvic floor.
• The sacrum is curved.
• The superior ramus is thinned and elongated with widening of the subpubic angle.
Second, the fetal head is able to negotiate the pelvis successfully because of three features:
1. Spheroid shape of the vertex.
2. Mobility of the head on the neck, allowing flexion or extension.
3. Moulding of the bones of the vault (Abitol 1993).
Cephalopelvic disproportion
Any condition leading to a misfit between the fetal head and the maternal pelvis, with failure of descent of the head into the pelvis despite good contractions, results in cephalopelvic disproportion (CPD). Ultimately CPD interferes with the natural mechanisms of labour. The presenting diameters of the fetal head are larger than the diameters of the pelvis (Neilson et al 2003). The shape of the pelvis may be abnormal but, as long as the diameters allow passage of the fetal head, delivery should follow as there should be no problem with the rest of the fetus. CPD is an absolute cause of obstructed labour and there are tremendous dangers for mother and fetus (Neilson et al 2003).
Diagnosis
In a primigravida it is expected that the fetal head should engage in the last 2–3 weeks of pregnancy. If the head does not engage, an attempt to make it engage is tried and, if unsuccessful, CPD should be suspected. The most common cause for non-engagement of the head is occipitoposterior position, with deflexed head and a presenting occipitofrontal diameter (11.5 cm). However, in most of these cases the head flexes and descent occurs in labour. Other causes of a non-engaged head include pelvic tumours, placenta praevia and polyhydramnios. A steep angle of inclination between the pelvic brim and the horizontal is found in some Afro-Caribbean women and may delay engagement until late in labour.
Maternal indications of possible CPD
These indications include:
• Bone conditions such as rickets or osteomalacia, which may have resulted in alterations in the size and shape of the pelvis (Neilson et al 2003).
• Spinal deformities such as scoliosis.
• Pelvic trauma and fractures which may have altered the size and shape of the pelvis.
• Previous obstetric conditions such as prolonged labour, difficult delivery or CS.
• Short stature of the woman. Mahmood et al (1988) found that the height of the woman was a better predictor of CPD than the shoe size, although 80% of women under 1.6 m still achieved vaginal delivery.
Fetal conditions leading to CPD
These conditions include:
• Fetal abnormalities such as hydrocephalus.
• Size of the fetus in relation to the maternal pelvis. In a multigravida with deliveries of normal-sized infants, CPD is less likely, but in the event of a larger fetus there may be a problem. Abdominal palpation is an inaccurate method of judging fetal size, although experienced practitioners may become quite adept. Estimation of fetal size is becoming easier as ultrasound technology advances, although Hofmeyr (2004) argues that neither clinical nor ultrasound examination is good at estimating fetal weight and ultimately diagnosing obstructed labour.
Assessing the pelvis
A combination of careful history taking and clinical expertise backed up by technology should enable selection of women at risk. Head fitting or pelvic assessment examinations may be carried out. However, Enkin et al (2000) write that there is:
reasonable correlation between clinical and radiological assessment of pelvic dimensions but neither is particularly accurate in predicting the outcome of labour and opinion varies about the value of pre-labour assessments.
Head fitting
In head fitting, the technique is to attempt to cause engagement of the non-engaged head. The woman is asked to empty her bladder and to lie flat on the examination couch. The symphysis pubis is located with the fingers of the right hand and the fetal head is held between the thumb and fingers of the left hand. The woman takes a deep breath and as she breathes out the head is pushed downwards and backwards into the brim of the pelvis. The fingers of the right hand palpate to assess whether the widest diameter of the head has entered the pelvic brim.
Pelvic assessment
Pelvic assessment of the shape and size of the pelvis is carried out by the obstetrician in the last few weeks of pregnancy if the head cannot be made to engage. The tissues will be softer, allowing ease of examination, and the fetus is large enough to relate to the size of the pelvis. The aim is to assess the brim, cavity and outlet of the pelvis. An attempt is made to measure the diagonal conjugate which runs from the lower border of the symphysis pubis to the sacral promontory and thus assess the anteroposterior diameter of the pelvic brim, also known as the true or obstetric conjugate, through which the fetus has to pass.
During a vaginal examination an attempt is made to reach the sacral promontory but in a good-sized pelvis it is unlikely to be reached as the diagonal conjugate measures 12–13 cm. If it is reached, 2 cm are subtracted to allow for the depth of the pubic bone and the obstetric conjugate is estimated. The size of the pelvic cavity is assessed by examination of the length and curve of the sacrum and by feeling the length of the sacrospinous ligament, which should accommodate two fingers.
Finally, the shape and size of the pelvic outlet can then be assessed. The ischial spines are located to see whether or not they are prominent, which may suggest a narrow transverse diameter of the outlet. The subpubic angle should be more than 90° and should accommodate the width of two fingers. One external measurement is made with the fist: the distance between the ischial tuberosities should accommodate a large fist.
X-ray pelvimetry
Erect lateral X-ray pelvimetry provides information about the size and shape of the pelvis and the relationship of the fetal head to the pelvic brim. However, there has been criticism of its use because of an association between prenatal irradiation and childhood leukaemia. It may also be a poor predictor of CPD and the results do not appear to affect the management; therefore, X-ray pelvimetry should seldom if ever be necessary in pregnancy (Enkin et al 2000). This is reiterated by Pattison & Farrell (1997) and Hofmeyr (2004), who inform us that in the four ‘randomised trials’ reviewed of 1000 women X-ray pelvi-metry led to more caesarean sections (CSs) but no increase in perinatal outcomes and the evidence does not support the use of pelvimetry in women with cephalic presentations. Pattinson & Farrell (1997) further stress that there were insufficient sample sizes to truly assess perinatal outcomes and more studies to evaluate this are required.
If X-ray pelvimetry is conducted, the following details can be noted:
• The shape of the pelvis.
• The shape of the sacrum.
• The inclination between the sacrum and pelvic brim.
• The anteroposterior diameters of the brim, cavity and outlet.
• The width of the sacrosciatic notch.
• The depth of the pelvic cavity.
Request for pelvimetry
Pelvimetry may be requested for the following:
• Any primigravida with the fetal head not engaged at term in whom clinical assessment suggests pelvic contraction.
• A primigravida with a breech presentation if external cephalic version has failed or is contraindicated and vaginal delivery is being considered.
• Any multipara with a history of difficult labour such as failure to progress in labour, prolonged labour and operative delivery, although these women should be offered pelvimetry in the postnatal period to avoid the risks of radiation to the fetus. A previous CS for any reason other than CPD is not a contraindication for trial of labour (Flamm et al 1994).
• Women with a history of injury or disease of the pelvis and spine or any limp or deformity.
A very small study ( n = 48) by Sporri et al (2002) examined the benefits of magnetic resonance imaging (MRI). More accurate measurements of the pelvic outlet without the danger of radiation may be achievable but due to the insufficient sample size no conclusion can be drawn from this evidence on labour outcome. More research is needed to explore the issue. Retrospective data analysis of MRI pelvimetric data in 781 women by Keller et al (2003) highlighted that women who had undergone a CS or assisted delivery had smaller pelvimetric dimensions compared to women who had a vaginal delivery. Depending on the antenatal findings discussed above, there are three possibilities: disproportion is not present and vaginal delivery will be possible; there is CPD of such a degree that vaginal delivery will not be possible; and there is a degree of CPD which may be overcome in labour.
Trial of vaginal delivery
If there are no obstetric or medical complications, the woman can be admitted to hospital for a trial of labour. The aim is to allow time for the contractions of labour, aided by the abdominal and pelvic floor muscles, to cause sufficient flexion and moulding of the fetal head so that descent occurs (Abitol 1993). Engagement of the head is likely to be followed by vaginal delivery. All primigravidae with a non-engaged head are considered to be undergoing a trial of labour (Brock 2004). An old but probably useful saying is that the fetal head is the best pelvimeter!
Selection of women for trial of vaginal delivery
• The presentation must be cephalic.
• There should be no major degree of CPD.
• The woman should be healthy with a good obstetric and medical history.
• There should be no pregnancy complications such as hypertension.
Management
There must be careful monitoring of mother and fetus and facilities for the immediate carrying out of a CS if needed. All observations are plotted on a partogram and any changes in the conditions of mother, fetus or progress noted by the midwife must be reported to the obstetrician, who is the decision maker (Shiers 2003). The obstetrician may wish to conduct all vaginal examinations. Ambulation and adoption of an upright position encourages flexion and descent of the head, maintenance of good uterine action and cervical dilatation (Simkin & Ancheta 2005).
Assessment of progress
Successful progression to a vaginal delivery should occur if the contractions are good, the fetal head flexes and the skull bones mould, the pelvic joints relax and maternal and fetal heart remain satisfactory. Progress is assessed by observation of descent of the fetal head by abdominal palpation. The dilatation of the cervix is assessed by vaginal examination. Progress in dilatation should follow that of 1 cm/h.
If progress is slow due to inefficient uterine action and thought to be unrelated to CPD, active management of labour can be undertaken and oxytocic drugs can be used. However, it is important to remember that the injudicious use of oxytocic drugs in the presence of more than a minor degree of CPD may lead to rupture of the uterus. If hyperstimulation occurs, the Syntocinon (oxytocin) infusion should be stopped immediately and the obstetrician informed. A CS may be necessary if:
• The progress of labour remains slow following the commencement of the oxytocin infusion.
• The head fails to descend in the presence of efficient contractions.
• Fetal distress arises.
Obstructed labour
Obstructed labour occurs when there is no advance of the presenting part despite strong uterine contractions (Shiers 2003). There is a large increase in maternal and fetal morbidity and mortality if labour is allowed to proceed in the presence of unrecognised obstructed labour. The situation is more common in remote areas of the world such as villages in Africa or India where women do not have access to trained personnel, but it can also occur in a developed country such as in the UK if a woman fails to disclose her pregnancy or to present herself for care in labour.
Causes
• Cephalopelvic disproportion is a cause of obstructed labour that is unresolvable except by CS (in remote areas of the world, division of the symphysis pubis—symphysiotomy—or a fetal destructive operation may save the life of the mother).
• Malpositions and malpresentations of the head, such as brow, posterior face or deep transverse arrest of the head.
• Fetal abnormalities such as hydrocephalus.
• Maternal tumours.
• Fibroids.
Signs and symptoms
Early signs
• There is little progress in labour, with no descent of the head despite efficient uterine action.
• On vaginal examination, the presenting part is high.
• The cervix dilates slowly and is not well applied to the presenting part (Brock 2004).
• The membranes have usually ruptured early and there is an ever-present risk of cord prolapse.
• In a primigravida there may be active phase arrest and the contractions stop for a while, finally restarting with increased strength. The woman may complain of severe and continuous pain.
• The multiparous woman may have tumultuous contractions that proceed rapidly to uterine rupture.
Late signs
• If nothing was done for the woman or, much more likely in the UK, if she presented herself for care late in labour, she may progress to having a raised temperature, rapid pulse and dehydration.
• On abdominal inspection, the uterus would appear to be moulded around the fetus because of tonic contraction and loss of liquor amnii.
• A Bandl’s pathological retraction ring may be seen as a ridge of tissue running obliquely across the abdomen. This denotes an extremely thinned lower uterine segment and imminent rupture of the uterus.
• There will be a cutting off of the fetoplacental blood supply and fetal oxygen supply and the fetus will die.
• On examination the vagina feels hot and dry and the presenting part is high. There may be excessive moulding and a large caput succedaneum obscuring the presenting part in a cephalic presentation.
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