Mifepristone versus placebo to increase the rate of spontaneous labour in people with a prior caesarean: a double-blind randomised controlled trial (Mi-labourTrial).

BACKGROUND: For people with a prior caesarean, avoiding a repeat caesarean and undergoing vaginal birth after caesarean (VBAC) may decrease both short- and long-term complications. Current options include awaiting labour or undergoing induction of labour (IOL) with mechanical cervical dilation followed by amniotomy and oxytocin infusion. Spontaneous labour is preferred as the use of exogenous oxytocin increases the rate of uterine rupture, a feared complication of VBAC. Additional options are vital to improve the chance of labour in people with a prior caesarean scar. Mifepristone, a progesterone receptor antagonist, may have utility in people with a prior caesarean. The medication occupies progesterone receptors on the myometrium, thereby making the uterus more receptive to endogenous contractile agents.

METHODS: The MiLabour trial is a double-blinded randomised controlled trial being carried out at two sites in New Zealand. People with a history of lower uterine segment caesarean birth are eligible. Participants are randomised in a 1:1 ratio to either a single dose of 200mg of mifepristone or to a single dose of placebo. The primary outcome of the MiLabour trial is onset of labour within 48 h of ingestion of study medication. To identify an increase in the rate of spontaneous labour within 48 h from 10 to 30% in the group receiving mifepristone, a minimum of 168 participants are required with a confidence interval of 95% and power at 90%.

DISCUSSION: Safely preventing repeat caesarean births is a health priority. If mifepristone is an appropriate option, this would greatly impact pregnancy care. Research in this area has been hampered by the well-founded fear of uterine rupture, a surgical emergency with the potential for morbidity and mortality. If mifepristone achieves a higher rate of labour without the need for oxytocin, this could become a standard treatment approach for people with a lower uterine segment scar as they approach their due date.

TRIAL REGISTRATION: The trial is registered on the Australian and New Zealand Clinical Trials Registry, anzctr.org.au. Full registry title is "Mifepristone versus placebo to increase the rate of spontaneous labour in people with a prior caesarean: A double blind randomised controlled trial (Mi-labourTrial)".

TRIAL ID: ACTRN12624000103561. Date registered 06/02/2024.