Cervical length at mid gestation for the screening of pre-term birth in twin pregnancies

Objective Short cervical length (CL) in mid-gestation is considered predictive of spontaneous preterm birth (PTB). The medical literature suggests 20 mm as the cut-off for high risk in twin pregnancies. Our objective was to assess the predictive value of CL for spontaneous PTB < 32 weeks' gestation in twin pregnancies and to calculate the cut-off point with the best sensitivity and specificity. Methods This was a single-center retrospective cohort study of women in whom CL had been measured by transvaginal ultrasound at 18–23 weeks' gestation. Pregnancies complicated by twin-to-twin transfusion syndrome, those requiring intrauterine therapy or those with indicated PTB were excluded. The predictive value of CL for PTB < 32 weeks was assessed. The distribution of CL measurements and the optimal cut-off in patients with PTB were calculated and logistic regression analysis was performed to assess the association between pregnancy characteristics and PTB. Results A total of 940 twin pregnancies were included. CL showed an area under the receiver–operating characteristics curve of 0.65 (95% CI, 0.58–0.71) for the prediction of PTB < 32 weeks. The optimal cut-off value for predicting PTB was 36 mm (sensitivity, 64.1%; specificity, 62.8%; positive predictive value, 13.5%; negative predictive value, 95.1%; accuracy, 62.9%). The relative risk of PTB with CL ≤ 36 mm was 2.35 (95% CI, 1.53–3.60; P < 0.001). After adjusting for confounders in logistic regression analysis, only CL (adjusted odds ratio (aOR), 0.94 (95% CI, 0.90–0.99); P = 0.03), and not monochorionicity (aOR 4.14 (95% CI, 0.89–19.25); P = 0.07), was independently associated with PTB. More than one-third (36%) of PTB cases delivering < 32 weeks had a normal CL in mid-gestation. This proportion rose to 85% when considering the 20-mm cut-off suggested by the medical literature. Conclusions This study shows that, despite the weak independent association, CL assessed in mid-gestation is a poor predictor of PTB < 32 weeks' gestation in asymptomatic twin pregnancies