‘The delivery of the head with or without forceps may have been quite easy … time passes. The child’s face becomes suffused. It endeavours unsuccessfully to breathe. Abdominal efforts by the mother or by her attendants produce no advance, gentle head traction is equally unavailing. Usually equanimity forsakes the attendants. They push, they pull. Alarm increases. Eventually “by greater strength of muscles or by some infernal juggle” the difficulty appears to be overcome, and the shoulders and trunk of a goodly child are delivered. The pallor of its body contrasts with the plum coloured cyanosis of the face and the small quantity of freshly expelled meconium about the buttocks. It dawns upon the attendants that their anxiety was not ill-founded. The baby lies limp and voiceless, and only too often remains so despite all efforts and resuscitation’.
W Morris
J Obstet Gynaecol Br Emp 1955;62:302
Definition
Shoulder dystocia simply means difficult delivery of the fetal shoulders, and is defined as a vaginal cephalic delivery that requires additional obstetric manoeuvres to deliver the fetal body after routine axial traction on the fetal head has been unsuccessful at completing the birth. Shoulder dystocia occurs when the anterior fetal shoulder impacts on the maternal symphysis pubis, or, less commonly, the posterior fetal shoulder impacts on the maternal sacral promontory.
Pathophysiology of Shoulder Dystocia
In the majority of women the antero-posterior (AP) diameter of the pelvic inlet is narrower than the oblique or transverse diameter. The fetal shoulders usually enter the pelvis in the oblique diameter. However, if the fetal bisacromial diameter is large and the fetal shoulders attempt to enter the pelvis in the narrower AP diameter, the anterior fetal shoulder will become impacted on the maternal symphysis pubis − this is shoulder dystocia ( Fig 12-1 ). On extremely rare occasions both shoulders may remain above the pelvic brim − bilateral shoulder dystocia; this requires considerable extension of the fetal neck and is usually associated with instrumental delivery.
Risk Factors for Shoulder Dystocia
There are a number of antenatal factors that increase the risk of shoulder dystocia, and in most of these the common denominator is fetal macrosomia:
Macrosomia
The greater the fetal birth weight the higher the risk of shoulder dystocia. A review of 175 886 vaginal births of infants born to non-diabetic mothers reported rates of shoulder dystocia of:
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5.2% in infants weighing 4001–4250 g
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9.1% in infants weighing 4251–4500 g
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14.3% in infants weighing 4501–4750 g
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29.0% in infants weighing 4751–5000 g.
Maternal Diabetes Mellitus
Maternal diabetes mellitus increases the risk of shoulder dystocia. Infants of diabetic mothers have a three- to fourfold increased risk of shoulder dystocia compared to infants of non-diabetic mothers for the same birth weight. These infants tend to have a higher shoulder-to-head circumference ratio, because the tissues that contribute to shoulder girth are insulin-sensitive and respond to hyperglycaemia and hyperinsulinism.
Instrumental Delivery
Compared to a spontaneous delivery shoulder dystocia is approximately twice as likely to occur with instrumental delivery.
Maternal Obesity
Shoulder dystocia is associated with obesity; however, obese women tend to have larger babies and the association may be due to fetal macrosomia, rather than maternal obesity per se .
Previous Shoulder Dystocia
Previous shoulder dystocia is a risk factor for recurrent shoulder dystocia. The rate of repeat shoulder dystocia in a subsequent vaginal delivery has been reported to be between 1.1% and 16.7%. The average recurrence rate is 10%, approximately 10 times the incidence in the general population. However, due to selection bias these recurrence rates may be under-estimated; elective caesarean section may be performed in some pregnancies following a previous birth complicated by shoulder dystocia.
A woman who has had a previous shoulder dystocia should be referred to a consultant-led antenatal clinic in subsequent pregnancies to discuss antenatal care and mode of delivery.
Intrapartum Risks
The risk of shoulder dystocia is increased in any labour in which progress is slow (prolonged first stage, prolonged second stage, use of oxytocin for augmentation of labour, instrumental delivery).
Prediction of Shoulder Dystocia
Antenatal detection of macrosomia is poor. Clinical fetal weight estimation is unreliable; third trimester ultrasound scans have at least a 10% margin for error for actual birth weight and sensitivity of just 60% for macrosomia (> 4.5 kg).
A retrospective review of 267 228 vaginal births reported that even the most powerful predictors for shoulder dystocia have a sensitivity of just 12% and positive predictive value of under 5%. The majority of cases of shoulder dystocia occur in women with no risk factors. Shoulder dystocia is, therefore, an unpredictable and largely unpreventable event. Maternity staff must always be alert to the possibility of shoulder dystocia with any vaginal delivery.
Prevention
Induction of Labour
In women with gestational diabetes the incidence of shoulder dystocia is reduced with early induction of labour. The NICE diabetes in pregnancy guideline recommends that pregnant women with diabetes should be offered delivery through induction of labour, or by elective caesarean section if indicated, after 38 completed weeks. This recommendation was made due to the increased risk of late stillbirth associated with diabetes; however, the intervention may also reduce this risk of shoulder dystocia in this cohort.
Induction of labour, however, does not prevent shoulder dystocia in non-diabetic women with a suspected macrosomic fetus and therefore should not be offered to non-diabetic women to reduce the risk of shoulder dystocia.
Caesarean Section
Infants of diabetic mothers have a two- to fourfold increased risk of shoulder dystocia compared with infants of the same birth weight born to non-diabetic mothers. A decision-analysis model estimated that in diabetic women with an estimated fetal weight (EFW) of greater than 4.5 kg, 443 caesarean sections would need to be performed to prevent one permanent brachial plexus injury (BPI). Therefore, National Guidelines in the UK and USA recommend the consideration of an elective caesarean section in pregnancies complicated by pre-existing or gestational diabetes if the EFW is greater than 4.5 kg.
It has been estimated that 3695 caesarean sections would be required to prevent one permanent BPI in the non-diabetic population. Therefore National Guidelines suggest that an elective caesarean section should only be considered if the EFW is above 5.0 kg, or there has been a previous severe shoulder dystocia, especially if it was associated with neonatal injury.
Management
Shoulder dystocia is unpredictable and therefore all maternity staff should have the skills required to manage the emergency. There are numerous techniques described that can be used to relieve shoulder dystocia. The Royal College of Obstetricians and Gynaecologists (RCOG) have published an evidence based algorithm for the management of shoulder dystocia ( Fig 12-2 ). No one manoeuvre is superior to another. The algorithm begins with simple measures, which are often effective, and leads progressively to more invasive manoeuvres.
Recognition of Shoulder Dystocia
Shoulder dystocia can occur following spontaneous or assisted delivery of the fetal head. The face and chin may be difficult to deliver and when the head does deliver it remains tightly applied to the vulva, retracts and depresses the perineum – the ‘turtle-neck’ sign. Restitution may not occur because the fetal head is so tightly applied to the perineum. When routine axial traction is applied to the fetal head the anterior shoulder fails to deliver.
Assistance Is Required
When shoulder dystocia is suspected or diagnosed help must be immediately summoned. This should include senior midwifery staff, the most experienced obstetrician available and a neonatologist. If shoulder dystocia is not resolved quickly then an anaesthetist should also be called.
State the Problem and Stop the Woman Pushing
The mother and her partner should be told that the baby’s shoulders are difficult to deliver, that additional help is required, and that someone will explain what is happening and what she needs to do. When help arrives, ‘shoulder dystocia’ should be clearly stated so that attendants immediately understand the problem to be managed. Maternal pushing should be discouraged as it will not resolve shoulder dystocia, and may increase the impaction of the shoulders.
McRoberts’ Position
McRoberts’ position is the most widely advocated first-line manoeuvre. This involves hyper-flexion of the maternal legs, which increases the relative AP diameter of the pelvis by straightening the sacrum relative to the lumbar spine and rotating the maternal pelvis cephalad. The success rate is reported to be between 40% and 90%.
To assume McRoberts’ position the mother should be laid supine with all pillows removed. An assistant on each side should then hyperflex her legs against her abdomen ( Fig 12-3 ). Apply routine traction to the fetal head (i.e. the same degree of traction as applied during a normal delivery) that is axial (i.e. in the axis of the fetal spine). If the shoulders are not released in McRoberts’ position by the application of routine, axial traction to the fetal head, traction should be stopped and an additional resolution manoeuvre attempted.
There is no evidence that using McRoberts’ position in anticipation of shoulder dystocia is helpful, therefore prophylactic McRoberts’ positioning is not recommended.
Suprapubic Pressure
Suprapubic pressure has two aims: (1) to reduce the bisacromial diameter of the fetal shoulders by adduction and (2) to rotate the shoulders into the wider oblique or transverse diameter of the maternal pelvis.
Whilst two assistants hold the woman in the McRoberts’ position, the third assistant should apply pressure superior to the maternal symphysis pubis. Pressure should be applied in a downward direction from the side of the fetal back (if this is known). The application of pressure from behind the fetal back will adduct the shoulders and rotate them into the wider oblique diameter of the pelvis ( Fig 12-4 ). If the side of the fetal back is unknown, suprapubic pressure should be applied from the side thought most likely to be the side of the fetal back, and if this pressure is not effective, pressure should be applied from the opposite side. If the shoulders are not released with a combination of McRoberts’ position, suprapubic pressure and routine axial traction, traction should be stopped and a different manoeuvre attempted.
