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Vaginal deliver rates after previous caesarean section have been on the increase through the 1980s and 1990s but even at the dawn of the 21st century, trial of scar remains a controversy in obstetric care.
Vaginal deliver rates after previous caesarean section have been on the increase through the 1980s and 1990s but even at the dawn of the 21st century, trial of scar remains a controversy in obstetric care. Generations of western obstetricians were brought up with the adage ‘once a caesarean, always a caesarean’ coined by Cragin in 19161, ironically in an effort to stress that one of the risks of a primary caesarean section was the more dangerous repeat operation that may be required as a consequence in the future.
In the early 20th century caesarean sections were performed through a vertical ‘classical’ incision extending from the lower segment up to the fundus. The introduction of the transverse lower segment incision by Kerr in the 1920s led to decline of the ‘classical’ operation. Several studies performed in the 1960s and 1970s2 along with larger studies performed in the 1980s3,4 confirmed the safety of vaginal delivery after previous caesarean section.
A large study comparing the risks of trial of labour to elective caesarean section found that the length of hospital stay, postpartum blood transfusion rate and incidence of postpartum fever were higher I the elective caesarean section group5. This finding was the result of the fact that 75% of the trial of labour group delivered vaginally. However, when the elective caesarean section group is compared to the failed trial of labour group, maternal morbidity is indeed higher with caesarean section following failed trial of labour6. In a well designed study involving 6,138 women McMahon et al6 reported increased morbidity amongst women with caesarean section following failed trial of labour compared to repeat elective caesarean section. There was increase incidence of uterine rupture, operative injury and hysterectomy, with the all complications rising twofold and major complications fivefold.
The success rates for vaginal delivery after caesarean section when all indications for the primary operation are considered, is approximately 75%3,4,6, approaching, in many hospitals the likelihood of a vaginal birth for a nulliparous patient. A meta analysis of 29 studies found that the success rate for vaginal delivery after caesarean ranged from 67% for patients with prior failure to progress in labour, to 85% where the indication for the primary operation was breech presentation, with all indication for previous caesarean section being associated with success rates of vaginal delivery that would make trial of labour appropriate7. Jongen et al8, in a study of 103 patients with prior caesarean section for failure of descent in the second stage, 80% managed to have a vaginal birth. The study group included 55 women with a previous failed attempt at instrumental delivery and 41 of these patients also achieved a vaginal birth.
The risk of uterine rupture is approximately 1% with a low transverse uterine incision, while it may be as high as 10% with a prior classical uterine incision and thus in the latter group trial of labour is generally contraindicated. The risk of uterine rupture following two transverse lower segment uterine incisions appears to be around 1.8% in a large study where the uterine rupture rate after one precious caesarean section was reported as 0.6%9. Data on the risk of uterine rupture is limited on patients with more than two previous cesarean sections but appears to be in the same region as that for patients with 2 previous caesarean sections. Large studies looking at prostaglandin gel110, and oxytocin11, have indicated that they are safe to use in women with a previous caesarean section. It would however be prudent to exercise caution when inducing or augmenting labour in patients with a uterine scar.
Complete uterine rupture involves all the layers of the uterus including serosa with complete or partial fetal extrusion in 50% of cases and carries significant maternal and fetal risks. Hysterectomy is required in 10% of cases of uterine rupture and although maternal deaths have been recorded, they are thankfully extremely rare. The principal fetal risk following uterine rupture is asphyxia, but uterine rupture per se, does not carry dire consequences for the fetus. Perinatal outcome appears to be linked to the time interval from the onset of the ominous FHR patterns to delivery of the infant and infants suffering permanent brain injury or death when this window exceeds 18 minutes12 with fetal heart rate abnormalities such as prolonged deceleration or bradycardia or repetitive severe variable decelerations providing the earliest indication of uterine rupture13. The risk of asphyxia related neurological injury due to uterine rupture appears to be approximately 1/2500 to 1/5000 trials of labour3,5,9.
Central to the debate of mode of delivery after previous cesarean section, are the prospective mothers’ wishes. Women are less likely to accept risks to their baby than their obstetricians and are no longer the passive recipient of care14. Maternal request figures more and more as an indication for repeat cesarean section and the informed consumer’s choice may still be a repeat cesarean section regardless of the likelihood and advocated benefits of a successful trial of vaginal delivery.
At present there is no method to predict which patients are likely to sustain uterine rupture which is the main risk of trial of scar and may occur even with optimum management of intrapartum care. In order to make trial of labour safer, it is important to have a high index of suspicion such that detection of uterine rupture is not delayed and early detection is followed by rapid intervention to improve maternal and fetal outlook. Neither vaginal delivery after cesarean section nor repeat cesarean section are without risks. In order to eliminate the risks of either, the way forward would be to reduce the primary cesarean section rate by revisiting the appropriateness of the indications of primary cesarean sections.
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