(571) Prediction of preterm birth in twin pregnancies using cervical length and baseline clinical variables

To develop a model to predict early delivery in twin pregnancies.

Study Design:
Secondary analysis of a clinical trial which randomized women carrying twin pregnancies and a cervical length <30mm to Arabin cervical pessary, vaginal progesterone, or placebo from 16 weeks 0 days through 23 weeks 6 days. We examined various endpoints for early delivery. For the primary analysis, a bootstrapped resampling method was used to develop and internally validate a Cox Proportional Hazards (PH) model for the outcome of gestational age at delivery as a continuous measure. Secondarily, logistic models to predict PTB at < 24, < 28, and < 32 weeks were created. Baseline enrollment factors associated with PTB < 32 weeks were assessed in all models. Predictors retained in at least 75% of the 1000 bootstrapped models were selected for the final models. A Kaplan-Meier curve was created to demonstrate the probability of remaining pregnant across GAs in weeks.

Results:

A total of 437 pregnancies were included. Variables predicting PTB in the final CoxPH model included cervical length (CL) (HR 0.66, 95%CI 0.54, 0.79), GA when qualifying CL was measured (HR 0.59 95%CI 0.49, 0.72), and the interaction between them (HR 1.02 95%CI 1.01, 1.03). Logistic models with predictors of CL and gestation at the time of CL measurement had moderate to excellent prediction of PTB prior to 24, 28 and 32 weeks (Table). The Kaplan-Meier curve illustrated the relationship between the covariates and GA at delivery. For example, median delivery GA if CL was < 15mm prior to 20 weeks GA was 23 weeks, but increased to 31 weeks if not less than 15mm until after 20 weeks (Figure, p< 0.001).

Conclusion:
Cervical length and the gestational age at diagnosis of shortened cervical length accurately predicted GA at delivery for twin pregnancies with a cervix <30mm, as well as preterm birth at GAs less than 32 weeks. Model performance was excellent when predicting very early preterm births at less than 24 weeks.