Re-evaluation of the subgroup analysis from the Royal College of Obstetricians and Gynaecologists randomized controlled trial of cervical cerclage

History-indicated cervical cerclage is offered to patients who are at risk of spontaneous preterm birth (SPTB), though the indications are controversial. A common practice of offering cerclage after three prior SPTBs or midtrimester losses (MTLs) is based on findings of the subgroup analysis of the 1993 Royal College of Obstetricians and Gynaecologists (RCOG) randomized trial of cervical cerclage. The subgroup analysis was performed by repeating the primary analysis within individual subgroups, which can lead to erroneous conclusions. We repeated the subgroup analysis by evaluating the interaction between the characteristic of interest and treatment allocation in a regression model. The interaction between cerclage and any prior PTB as a binary variable was non-significant. Among subjects delivering at <37 weeks, there was a significant interaction between cerclage and prior PTBs as a continuous variable or >= 3 (p-values 0.04 and 0.03, respectively). There were no significant interactions between cerclage and the aforementioned outcomes among women who delivered at <33 weeks, though this may have been secondary to a smaller number of SPTB in this range. Our findings lend credence to the current recommendations regarding the use of history-indicated cerclage, though they remain subject to the inherent limitations of subgroup analyses.