‘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.