SHOULDER DYSTOCIA RISK FACTORS
Risk factors associated with the development of shoulder dystocia during labour include:
- large foetus
- small maternal pelvis
- maternal obesity
- diabetes mellitus
- post maturity
- previous shoulder dystocia
- prolongation of the late first stage of labour
- prolonged second stage of labour
- assisted vaginal delivery
The term ‘macrosomia’ is used to describe a newborn with an excessive birth weight therefore; a diagnosis of foetal macrosomia can only be made after a baby has been delivered. An excessive birth weight is one greater than 4.0kg (although the range of macrosomia is between 4.0kg and 4.5kg; compare that with the normal birth weight of between 2.5kg and 3.9kg).
Poorly controlled maternal diabetes mellitus, maternal obesity and excessive maternal weight gain are all associated with macrosomia because they all have one thing in common – HYPERGLYCEMIA. The increased serum concentration of glucose in the maternal blood passes into the foetal circulation. This increased serum glucose concentration in the foetal blood stimulates the release of foetal insulin, insulin-like growth factors, growth hormone and growth factors that stimulate foetal growth as well as the deposition of fat and glycogen in foetal tissues. As the pregnancy advances, the periods of hyperglycemia to which the foetus is exposed to increases; this results in an increase in the size of the foetus.
In macrosomia cases the maternal pelvis (or birth canal) may be inadequate to admit the large baby and it is therefore not difficult to see why the shoulders may become caught up above the pelvic inlet. Macrosomia is associated with an increased risk of intrauterine death (from infection) and birth trauma for both baby (brachial plexus injuries) and mother (perineal, vaginal and cervical lacerations may all occur).
It is important to note that macrosomia could be the result of a genetic component (for instance, Saudi Arabian women are known to deliver big and healthy babies) – so macrosomia doesn’t always have to be part of an ongoing disease process as in the case of poorly controlled diabetes mellitus.
Pelvimetry involves estimating the dimensions of a gynecoid (or female) pelvis. The normal pelvic inlet has an AP (antero-posterior) diameter of 11cm and a transverse diameter of 13.5cm; the mid pelvis has an AP diameter of 12cm and a transverse diameter of 12cm; and the pelvic outlet has an AP diameter of 13.5cm and a transverse diameter of 11cm. If for any reason the dimensions of a female pelvis fall below the above, the pelvis becomes less than ideal for childbirth and there is an increased risk of shoulder dystocia occurring during delivery even in the absence of macrosomia.
It is necessary to distinguish between prolonged pregnancy and post maturity. Prolonged pregnancy is defined as pregnancy exceeding 42 completed weeks of gestation or pregnancy exceeding 14 days from the normal length of 280 days (note that the first day of a pregnancy is the first day of the last normal menstrual period). Post maturity is a syndrome associated with meconium stained liquor, oligohydramnios and observational loss of subcutaneous fat with dry, cracked skin of the baby following delivery. The syndrome of post maturity can occur in a pregnancy less than 42 completed weeks of gestation and diagnosis is best made after delivery.
Post maturity is a factor that has been known to be associated with shoulder dystocia.
There is a risk of recurrence of shoulder dystocia in subsequent pregnancies although definite figures are not available.
Prolonged Second Stage Of Labour
Labour occurs in three stages – stage one begins with painful, regular and progressive uterine contractions and ends with cervical effacement and dilation up to 10cm; stage two ends with the delivery of the foetus and stage three ends with delivery of the placenta.
Failure of the foetal head to descend during the second stage of labour; and a lengthy second stage of labour are associated with shoulder dystocia.
Shoulder dystocia has been found to be more common in instrumental deliveries than in spontaneous vaginal deliveries.