Vaginal birth after caesarean section (VBAC, previously called ‘trial of scar’) is the term used for the trial of labour in a woman who has a scar on her uterus, usually from a lower- segment caesarean section, but also from a previous hysterotomy or myomectomy.
Problems and special considerations
Risk factors for unsuccessful VBAC are induction of labour, increased maternal age, high body mass index (BMI) and no previous vaginal delivery. A previous vaginal delivery is the single biggest predictor of success.
A trial of scar would be considered if the reason for the scar was not a recurrent obstetric problem, such as cephalopelvic disproportion. The major anxiety is rupture of the uterine scar, particularly during strong uterine contractions. The incidence of uterine rupture in women undergoing a planned VBAC is around 0.2-0.5%, although it is substantially higher after a vertical uterine incision (4-9%), which is seen as a contraindication to VBAC. The risk is thought to be increased if prostaglandins are used for the induction of labour, although oxytocin, which is more controllable, is not usually considered contraindicated.
Uterine rupture may present with an abnormal fetal heart trace in up to 76% of cases. Other associated features may include abdominal pain, tenderness over the uterine scar, maternal haemodynamic compromise, intrapartum bleeding or cessation of labour. If uterine rupture occurs, this is an obstetric emergency and urgent delivery is required.
There is a 25-30% likelihood of a repeat caesarean section if the reason for the previous caesarean section is non-recurrent.
There have been anxieties that epidural analgesia may mask the pain of uterine dehiscence. However, pain is not a constant feature of uterine rupture and may be absent in some cases. In addition, severe pain may be present in the absence of uterine rupture. Finally, the pain of uterine rupture has been reported to ‘break through’ analgesia provided by modern, low-dose epidural techniques. In fact, many would consider epidural analgesia indicated in VBAC, since it may be readily converted to anaesthesia suitable for caesarean section if required (unless there is uterine rupture, in which case there may not be time to extend the epidural).
Management options
Women undergoing VBAC (and often, their obstetricians) should have the potential advantages and disadvantages of regional analgesia explained to them. Pain that breaks through low-dose epidural analgesia or is present between contractions should raise the possibility of uterine dehiscence.
Key points
• Uterine rupture is the most important complication of VBAC and occurs in ~2-5 cases per 1000.
• Epidural analgesia is not contraindicated, and early insertion is recommended.
• Pain in the presence of a working epidural may be a warning of impending uterine rupture.
Further reading
Cahill AG, Odibo AO, Allsworth JE, Macones GA. Frequent epidural dosing as a marker for impending uterine rupture in patients who attempt vaginal birth after cesarean delivery. Am J Obstet Gynecol 2010; 202: 355.e1-5.
Guise JM, Denman MA, Emeis C, et al. Vaginal birth after cesarean: new insights on maternal and neonatal outcomes. Obstet Gynecol 2010; 115: 1267-78.
Royal College of Obstetricians and Gynaecologists. Birth after Previous Caesarean Birth. Green-top Guideline 45. London: RCOG, 2015. www.rcog.org.uk/en/guidelines-research-services/guidelines/ gtg45 (accessed December 2018).
Scott JR. Intrapartum management of trial of labour after caesarean delivery: evidence and experience. BJOG 2014; 121: 157-62.