Definitions
It is important that obstetric anaesthetists understand the terminology used by their obstetric and midwifery colleagues:
• Lie: the relationship of the long axis of the fetus to that of the mother, e.g. longitudinal, transverse, oblique
• Presentation: the part of the fetus that is foremost in the birth canal, e.g. cephalic, breech or compound
• Position: the relationship of the presenting part of the fetus, using a reference point such as the occiput or sacrum, to the maternal pelvis, e.g. left occipito-anterior (LOA) or right sacral transverse (RST)
Approximately 85% of fetuses at term lie longitudinally, with a cephalic presentation in an occipito-anterior position. A malpresentation is anything that does not fulfil these criteria.
Problems and special considerations
The malpresenting fetus is less likely to deliver spontaneously, and instrumental or operative intervention is often required. Labour is often prolonged and particularly painful. Although it has been suggested that epidural analgesia may increase the likelihood of malpresentation through relaxation of pelvic musculature, impeding rotation, there is little evidence to support this view.
Occipito-posterior position
This is the commonest malpresentation, occurring in over 10% of term pregnancies - though in only 5% of vaginal deliveries, as many turn during labour. Progress of labour and descent of the head may be slow, and the mother often experiences particularly severe pain in the back, which may be resistant to treatment by regional blockade. Manual or instrumental rotation may be attempted to bring the head into a more favourable occipito-anterior position. The likelihood of operative delivery and perineal trauma may be increased.
Breech presentation
This occurs in 3-4% of term pregnancies and can be subdivided into:
• Frank (hips flexed and legs extended over abdominal wall)
• Complete (hips and legs flexed)
• Footling (foot or knee presenting)
The mother with a breech presentation may get the urge to ‘push’ before the cervix is fully dilated, thus running the risk of trapping the fetal head; this is a particular risk if the labour is preterm. It is becoming increasingly common for women with breech presentation to be delivered by elective caesarean section, especially if primiparous, as this reduces neonatal short-term morbidity and mortality. Perinatal mortality is 2 in 1000 with vaginal breech delivery, 1 in 1000 with vaginal cephalic delivery, and 0.5 in 1000 with caesarean section after 39 weeks. However, appropriate selection of pregnancies and skilled intrapartum care may allow planned vaginal breech delivery to be nearly as safe as planned vaginal cephalic delivery. Maternal complications are least with a vaginal breech delivery, but 40% of planned vaginal breech deliveries will end in emergency caesarean section. External cephalic version (ECV) is routinely offered to women with breech presentation; in this manoeuvre, the obstetrician applies external pressure to rotate the fetus to a vertex (cephalic) presentation (see Chapter 67, External cephalic version).
Transverse lie
This occurs in 0.3% of term pregnancies and may be associated with placenta praevia, polyhydramnios and grand multiparity. There is an increased risk of cord prolapse. Spontaneous delivery is impossible unless the lie is converted to longitudinal, which may be achieved by external version provided that placenta praevia has been excluded. Caesarean section is usually necessary, and a vertical uterine incision may be needed to aid delivery.
Face and brow presentations
These are rare presentations, where the head is hyperextended. They are associated with multiparity, prematurity, polyhydramnios and fetal abnormality. A face presentation may deliver vaginally, but caesarean section is often needed.
Prolapsed cord
Cord prolapse occurs in 0.4% of cases when the head is presenting, but this incidence rises to 0.5% in frank breech, 4-6% in complete breech and 15-18% in footling presentations. It is generally more common when the fetus does not fully occlude the pelvic inlet, as in preterm labour, and may follow artificial rupture of the membranes with a high presenting part. If immediate vaginal delivery is not feasible, the presenting part is pushed and held out of the pelvis to prevent cord compression, often aided by steep head-down tilt, while the mother is transferred to theatre for immediate caesarean section. (See Chapter 73, Cord prolapse.)
Management options
Good regional analgesia is desirable at an early stage, since intervention is more likely to be required. If there is breakthrough pain, for example with an occipito-posterior position, addition of an epidural opioid such as fentanyl may improve pain relief, although more concentrated solutions of local anaesthetic than those used in ‘low-dose’ techniques may be required.
If vaginal delivery of a breech presentation is planned, epidural analgesia will help prevent premature ‘pushing’ and will enable controlled manipulation, extensive episiotomy and application of forceps to the after-coming head. If head entrapment occurs, the anaesthetist may be asked to relax the uterus, for example by using sublingual glyceryl trinitrate. Delivery will often occur in the operating theatre, where an emergency operative delivery can be provided.
For cord prolapse requiring caesarean section, general anaesthesia is usually the quickest option, although extension of a pre-existing epidural block or institution of spinal anaesthesia is also possible (see Chapter 73, Cord prolapse).
Key points
• Regional analgesia is particularly indicated in malpresentation.
• Prolapsed cord is often associated with breech presentation and transverse lie, and with preterm delivery.
• Early multidisciplinary communication will help optimise management.
Further reading
Hannah ME, Hannah WJ, Hewson SA, et al. Planned caesarean section versus planned vaginal birth for breech presentation at term: a randomised multicentre trial. Term Breech Trial Collaborative Group. Lancet 2000; 356: 1375-83.
Impey LWM, Murphy DJ, Griffiths M, Penna LK; Royal College of Obstetricians and Gynaecologists. External cephalic version and reducing the incidence of term breech presentation. Green-top Guideline 20a. BJOG 2017; 124: e178-92.