Objective

The effectiveness of preterm birth (PTB) prevention by cerclage placement in patients with twin pregnancies and a short or dilated cervix represents a clinical issue associated with marked uncertainty. A reanalysis of a systematic review and meta-analysis of randomized controlled trials (RCTs) concluded that cerclage placement in patients with twin gestations with a sonographic short cervix is associated with an increased risk of preterm birth and adverse perinatal outcomes. Several systematic reviews and meta-analyses have reached similar conclusions. A more recent RCT, however, suggested that physical examination-indicated cerclage in twin pregnancies significantly decreased preterm birth at all gestational age cutoffs and reduced the risk of perinatal morbidity and mortality. Given the importance placed on RCTs in the hierarchy of evidence to inform the development and conclusions of clinical obstetrical practice, we assessed the fragility of the RCT indicating the beneficial effects of cerclage placement in twin pregnancies with a short or dilated cervix.

Study Design

Roman et al performed a multicenter RCT, aimed to determine whether physical examination-indicated cerclage reduced the incidence of preterm birth in patients with twin pregnancies and asymptomatic cervical dilation before 24 weeks of gestation. The primary outcome was the incidence of spontaneous PTB at <34 weeks of gestation.

We assessed the fragility index (FI) of the Roman et al trial using the method described by Walsh et al. The FI was calculated by changing the status of patients from a “non-event” to an “event” outcome in the treatment group with the smallest number of events until the P value (using the Fisher’s exact test) exceeded .05. The FI score represents the number of patients responsible for the statistical significance of a trial finding, and it is an intuitive measure of the robustness of an RCT.

Results

Roman et al randomized and analyzed 30 patients (17 were allocated to the cerclage group, and 13 did not undergo cerclage; ultimately, 3 of these 30 individuals were excluded after randomization). The authors concluded that cerclage placement was associated with a statistically significant reduction in PTBs at all gestational age cutoffs ( Table ). Although cerclage placement was associated with a significant reduction in the primary outcome (relative risk [RR], 0.71; 95% confidence interval [CI], 0.52–0.96; P value, .05), the FI score was 0 to obtain a P value >.05 and was 2 to obtain a 95% CI that includes null).

Table

Results of effect estimates and their fragility assessment for the study of Roman et al

Outcome	RR (95% CI)	P value	Fragility index a	Fragility index b	Fragility quotient	NNT (95% CI)
PTD <34 wk	0.71 (0.52, 0.96)	.05	0	2	0	3.4 (1.8–21.5)
PTD <32 wk	0.65 (0.46, 0.92)	.02	1	3	0.07	5.7 (−32.5 to 2.6)
PTD <28 wk	0.49 (0.26, 0.89)	.02	1	2	0.07	3.2 (−235.1 to 1.6)
PTD <24 wk	0.35 (0.16, 0.75)	.004	3	4	0.13	3.2 (−235.1 to 1.6)

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