‘FAULTS’ IN THE FETUS (THE PASSENGER)

The presentation of the fetus may cause prolongation of labour (for example an occipitoposterior position), as may the size of the fetus. A large fetus (weight >4000 g) may not easily be able to be born vaginally even if the pelvis is normal in size. Some genetically programmed mothers habitually produce large babies, as do some diabetic mothers. Some babies have congenital defects that may make vaginal births difficult or impossible, for example a fetus who has hydrocephaly, or a tumour of the neck or abdomen. A fault in the fetus may be anticipated if an abdominal examination in late pregnancy reveals a large fetus whose head has not entered the maternal pelvis. The diagnosis can be established by an ultrasound examination.

ABNORMAL SHAPE OR SIZE OF THE PELVIS (THE PASSAGES)

The ideal obstetric pelvis is described on page 55. If any of the two main diameters, particularly of the pelvic brim, is reduced by 2 cm or more the pelvis is considered to be contracted. The shape of the pelvis may also be affected, for example the sacral curve may be replaced by a straight sacrum, or the pelvis may have been damaged by a serious accident.

Major pelvic abnormalities are uncommon in developed countries. The final arbiter of a successful vaginal delivery is the quality of the uterine contractions, the degree of relaxation of the pelvic ligaments and the moulding of the fetal head. Pelvimetry has little place in modern obstetric practice.

In the developing nations pelvimetry, preferably by CT scan (if available), still has a place to enable the obstetrician to make a decision about a vaginal delivery, but this prognosis is not particularly accurate, as different observers attach different prognostic values to the measurements and the shape of the pelvis. For example, if the anteroposterior diameter of the pelvic brim is less than 9 cm nearly all obstetricians will perform a caesarean section, whereas if the anteroposterior diameter is between 9 cm and 10.5 cm an obstetrician may attempt a trial of labour, provided the woman agrees, unless the sacrum is straight, when a caesarean section will be performed. If the anteroposterior diameter is more than 10.5 cm a vaginal delivery will be anticipated, unless the fetal head is very large.

CEPHALOPELVIC DISPROPORTION

Apart from the quality of the uterine contractions, the main issue with regard to vaginal birth is the size of the fetus relative to the size of the maternal pelvis; thus the concept of cephalopelvic disproportion (CPD) has arisen. The term cephalopelvic is used rather than fetopelvic, as any presentation other than cephalic would be managed by caesarean section.

In cases of CPD, if the fetal head has not entered the pelvic brim by term a caesarean section is likely to be performed because of risks to the fetus should labour proceed. On the other hand, if the fetal head has entered the pelvic brim the choice is between an elective caesarean section and a trial of labour. The decision depends on the woman’s preference and the doctor’s experience. For example, a woman over the age of 35, or one who has had a long history of infertility, will generally be delivered by caesarean section, as will women with medical complications.

If a trial of labour is attempted, the patient should be told that she has about a 30% chance that she will require an instrumental vaginal delivery and a 30% chance that the trial will be abandoned and a caesarean section will be performed.

In a trial of labour the aim is to determine what the woman can accomplish, not how much she can endure. As operative delivery is likely, an intravenous infusion should be established and only fluids permitted by mouth. The progress of the labour is monitored using the partogram, particular attention being paid to the speed of the descent of the fetal head and the dilatation of the cervix. A vaginal examination is made when the membranes rupture, to exclude cord prolapse. Special attention is paid to the woman’s response to the trial. The trial should be abandoned if:

ABNORMALITIES OF UTERINE ACTION

Labour will only progress normally if the contractile wave is propagated over the entire uterus in a triple descending gradient of activity (see p. 61).

If the normal pattern of uterine activity fails to occur the progress of labour will be abnormal – usually prolonged. Until the 1940s, prolonged labour was considered to be caused by ‘uterine inertia’. Research since then has shown that several patterns of uterine activity may lead to delay in the birth of the child. The patterns are designated inefficient uterine activity and are divided into subgroups of abnormal uterine activity (Box 22.1). In some cases of labour the reverse occurs and the uterus is overactive, leading to a precipitate birth.

Changes in the management of labour in recent years, with the increasing use of epidural anaesthesia and of caesarean section, have rendered the description of the patterns of uterine activity less useful. They are worth recording, however, as they form the physiological basis of the modern classification.

Types of abnormal uterine action

Hypoactive states (uterine inertia)

The uterine resting tone is low and the intensity of the contraction is reduced, with the result that only a feeble contractile wave is propagated (Fig. 22.1). The contractions occur at longer intervals than usual and do not cause the patient much distress.

Fig. 22.1 Hypoactive uterus. Note that the intensity of the contraction is diminished and the tone lowered slightly.

Hyperactive incoordinate states (incoordinate uterine action)

In normal labour the perception of pain is usually only reached when the uterine tone exceeds 25 mmHg. In hyperactive, incoordinate uterine activity the resting uterine tone is increased; in consequence, the pain threshold is reached earlier during the contraction and the pain persists for longer (Fig. 22.2). In spite of the strong contractions, cervical dilatation is slow because the triple gradient is reversed (Fig. 22.3).

Fig. 22.2 Pain in incoordinate uterine action. Because the resting tone is raised, the pain threshold is reached earlier in the contraction and remains above the threshold for longer.

Fig. 22.3 Hyperactive lower uterine segment, with reversal of the normal gradients of activity. In this case the uterine tone is still normal but the cervix fails to dilate.

If the woman has not been given an epidural anaesthetic she may complain of severe backache, which increases during each contraction when the pain radiates into her lower abdomen.

Two variations of hyperactive uterine activity may occur. Both are uncommon today and are the result of action not being taken to relieve the problem. They are colicky uterus and constriction ring dystocia. In colicky uterus, various parts of the uterus contract independently and the pain of the contraction is generalized and severe. Constriction ring dystocia occurs when an annular spasm arises at the junction between the upper and the lower uterine segments, usually late in the first stage of labour or early in the second stage. It may follow the injudicious use of oxytocin or intra-uterine manipulations.

The modern classification of inefficient uterine activity

As the clinical diagnosis of the two main types of inefficient uterine activity depends on the patient’s perception of the severity of the pain, its distribution and the findings on uterine palpation, the increasing use of epidural anaesthesia has limited the value of the classification.

With the introduction of the partogram, a different classification has been developed (Fig. 22.4).

• Prolonged latent (quiet) phase. The duration of the latent phase of labour is increased. In most cases the pattern of uterine activity shows hypoactivity.

• Prolonged active phase. This can be equated to the hyperactive incoordinate state of uterine activity, usually involving a hyperactive uterine lower segment (i.e. a reverse gradient of uterine activity).

• Secondary arrest of cervical dilatation. This pattern can be equated to incoordinate uterine activity or to the end result of obstruction to the birth in a nulliparous woman.

Fig. 22.4 Abnormal patterns of cervical dilatation.

UTERINE OVEREFFICIENCY

SHOULDER GIRDLE DYSTOCIA

Shoulder dystocia is frequently unanticipated as it can occur in the absence of known risk factors which include maternal diabetes, macrosomia, maternal obesity, prolonged second stage, a rotational operative vaginal delivery, or previous shoulder dystocia. The fetal head may be born but the head burrows back into the perineum (turtle sign) as the bisacromial diameter of the fetal shoulders fails to rotate to enter the transverse diameter of the pelvic brim. The baby must be delivered quickly or it will die (Fig. 22.5A–D). Shoulder girdle dystocia occurs in 1% of births and the delivery requires considerable experience: damage to the lower genital tract is a usual occurrence, and damage to the baby not uncommon. Most often the damage is to the brachial plexus, or the clavicle or humerus can be fractured. The latter two will heal without sequelae, but a brachial plexus palsy can be permanent. Only 50% of brachial plexus palsies actually follow shoulder dystocia and some 5% occur in association with a caesarean delivery which implies that at least some have an antepartum aetiology.

Fig. 22.5 (A) The accoucheur’s hand is inserted into the vagina and digital pressure is applied to the posterior aspect of the anterior shoulder pushing it towards the fetal chest. This rotates the shoulders forward into the more favourable oblique diameter. Attempt delivery. (B) While maintaining pressure as for Rubin manoeuvre, the accoucheur introduces their second hand and locates the anterior aspect of the posterior shoulder. Pressure is applied to rotate the posterior shoulder. Attempt delivery once the shoulders move into the oblique diameter. If this movement is unsuccessful, continue rotation through 180° and attempt delivery. (C) Apply pressure to the posterior aspect of the posterior shoulder and attempt to rotate it through 180° in the opposite direction to that described in the Wood screw manoeuvre. (D) The accoucheur passes their hand into the vagina over the chest of the fetus to identify the posterior arm and elbow. Apply pressure to the antecubital fossa to flex the elbow in front of the body, and/or grasp the posterior hand to sweep the arm across the chest and deliver the arm. This is followed by rotation of the fetus into the oblique diameter of the pelvis, or through 180°, bringing the anterior shoulder under the symphysis pubis.

PROLONGED LABOUR

By convention, a labour which has lasted more than 12 hours after the woman has been admitted to hospital in labour is considered to be prolonged. Between 5 and 8% of labours are classified as prolonged, and the incidence is twice as common among nulliparous women than among multipara. Cephalopelvic disproportion and inefficient uterine action are the usual causes of prolonged labour, but in a few cases no cause can be identified.

Fifty years ago, labour was defined as being prolonged if it had lasted for more than 48 hours. Labours of this length were associated with increased maternal morbidity. During these labours dehydration often occurred, infection was not uncommon, and the risk of postpartum haemorrhage was increased. These problems should not arise in the developed countries today, but they still occur in some developing countries. Perinatal morbidity and mortality also increase in prolonged labour. The main causes are pneumonia following intra-uterine infection, increased fetal acidaemia, and trauma from an operative delivery.

Earlier intervention has reduced or eliminated these problems, but care must be taken to monitor women in labour to ensure they do not become dehydrated, ketoacidotic or infected. If the woman is anxious and distressed this should be attended to by reassurance and explanation of the progress of the labour.

PSYCHOLOGICAL EFFECTS OF DYSTOCIA AND PROLONGED LABOUR

The memory of a difficult labour, attended by unsympathetic, uncommunicative medical personnel and terminated by an operative delivery, may leave a scar on the woman’s mind deeper than the scar on her abdomen or perineum.

The woman and her partner need explanation and reassurance. Most couples are anxious to know whether the next childbirth will be as difficult and painful as the last one. As inefficient uterine action is the only identified cause in many cases of prolonged labour, and as this is less likely to occur in a subsequent labour, the woman can be given some reassurance. When she becomes pregnant again, the doctor or midwife responsible for her antenatal care needs to listen to her concerns, to reduce her anxieties and to communicate with her, so that she reaches labour in a positive frame of mind. The use of adequate anaesthesia should be discussed and the patient given the choice of having an epidural anaesthetic if she so wishes.

OBSTRUCTED LABOUR

Obstructed labour is the end result of a poorly managed or neglected labour, in which CPD or a shoulder presentation has not been detected early and timely intervention effected.

During an obstructed labour, uterine contractions attempt to overcome the obstruction. In a first labour the uterus contracts strongly for a while and then, failing to overcome the obstruction, becomes hypoactive, developing secondary arrest. In contrast, if the obstruction occurs in a subsequent labour, the uterus continues to contract strongly in an attempt to push the fetus through the maternal pelvis. With each contraction there is some myometrial shortening (retraction), the upper uterine segment becoming progressively thicker and shorter, and the lower segment becoming progressively stretched and thinner. The junction between the two segments becomes obvious, forming a pathological retraction ring – Bandl’s ring (Fig. 22.6). The pathological retraction ring may be confused with a distended urinary bladder, but the oblique line is diagnostic. The patient becomes dehydrated, with a coated tongue and dry lips. She has tachycardia and concentrated urine, and faces the risk of a ruptured uterus at any time.

Fig. 22.6 Threatened rupture of the uterus in a case of cephalopelvic disproportion and obstructed labour.

Treatment is urgent. The dehydration should be corrected rapidly and a caesarean section performed under antibiotic cover as soon as possible, even if the fetus is dead.

RUPTURE OF THE UTERUS

The end result of obstructed labour, unless intervention is made, is rupture of the uterus. Rupture may also occur in late pregnancy, when it may follow trauma to the uterus, or derive from a caesarean scar. In labour, as well as following an obstructed labour (Fig. 22.7), rupture of the uterus may be caused by the inappropriate use of oxytocics and the dehiscence of a caesarean section scar.

Fig. 22.7 Threatened rupture of the uterus in a neglected shoulder presentation.

In modern obstetric practice the most common cause of uterine rupture is dehiscence and extension of a previous caesarean scar. Three per cent of classic scars rupture, most of them during labour, which is the reason for performing an elective caesarean section on a woman who has a classic caesarean scar. Rupture of a lower segment caesarean scar is less common, 0.5% rupturing, mostly during labour.

In cases of uterine rupture following trauma or obstructed labour, the rupture usually involves one or other lateral wall and extends into the upper uterine segment (Fig. 22.8).

Fig. 22.8 Ruptured uterus, from a case of obstructed labour.

Rupture of a lower segment caesarean scar is usually quiet. The first sign is most commonly changes in the fetal heart pattern and for this reason all such labours should be monitored by cardiotocography (CTG). Slight vaginal bleeding may occur and the patient complains of pain, which becomes constant. The patient’s pulse begins to rise, and in some cases shock supervenes. In some patients no symptoms or signs occur and the ruptured uterus is only detected by intra-uterine palpation following the birth.

CLASSIFICATION OF PELVIC ABNORMALITIES

In the developed nations of the world, contracted pelvis has diminished as a cause of dystocia. However, among the less privileged groups in the rich nations and among the urban poor in the developing nations, contracted pelvis still occurs and may lead to a difficult labour.

Major defects of nutrition or environment

Rickets

In the past 50 years, in most developed countries rickets has largely disappeared as increasing attention has been given to social and preventive medicine. Because in many of the developing countries they have an abundance of sunshine (which converts ergosterol in the skin into vitamin D), and most of the drinking water is rich in calcium, rickets is not common among children.

In a young child suffering from rickets the weight of the upper body presses down through the spine on to the softened pelvic bones. The sacral promontory is pushed forwards and downwards, and the sacrum itself pivots backwards. At the same time, the ligaments of the back draw the spinous process medially and the ilia flare outwards, as do the ischial tuberosities. In extreme cases the softened acetabula may be forced inwards. The main alteration in pelvic shape is a marked reduction of the anteroposterior measurement of the brim, with some irregular widening of the cavity (Fig. 22.9).

Fig. 22.9 The rachitic pelvis. Tracing from a radiograph showing the anteroposterior diameter of 8 cm, the widest transverse diameter of 11.7 cm and a brim index of 69. The arrows show how pressure on the softened bones alters the shape of the pelvis.

Osteomalacia

This is due to an acquired deficiency of calcium and, encountered in adult life, causes the same deformities as rickets, but is rare, except in certain inland parts of northern India and China (Fig. 22.10).

Fig. 22.10 Osteomalacic pelvis. The arrows show how pressure deforms the softened bones.

Disease or injury

Spinal

Kyphosis of the lower dorsal or lumbar region that started in childhood may alter the shape of the brim of the pelvis, as the weight of the body pushes the upper part of the sacrum backwards and the lower part forwards. The side walls of the pelvis converge, forming a funnel. In scoliosis the altered pressure distribution on the soft bones may cause bays on each side of the sacral promontory, rendering the shape of the brim asymmetrical (Fig. 22.11). Spondylolisthesis, which means the slipping forward of the fifth lumbar vertebra to project beyond the sacral promontory, is rare (Fig. 22.12).

Fig. 22.11 Scoliosis and kyphosis.

Fig. 22.12 Spondylolisthesis. Tracing from the radiograph showing the subluxation of the fifth lumbar vertebra upon the sacrum. The diagnosis can only be made by radiography.

Pelvic

Osteomata may develop, or an injury may lead to excessive bone formation over the site of the fracture.

Limbs

Poliomyelitis in childhood, or congenital dislocation of the hip, may cause pelvic deformity. The child puts most of its weight on the stronger leg, and on this side the pelvis is pressed in, with flattening of the brim on the same side.

Congenital malformations

These include Naegele’s pelvis and Robert’s pelvis, and are due to the defective development of one or both sacral lateral masses, causing the sacrum to fuse with the ilium on one or both sides (Fig. 22.13). The high assimilation pelvis occurs when the fifth lumbar vertebra is fused to the sacrum, thus increasing the inclination of the pelvic brim. This hinders engagement of the fetal head.

Fig. 22.13 Naegele’s pelvis. Note the absence of the sacral alae.