To investigate the accuracy with which transvaginal cervical sonography predicts spontaneous preterm birth in twin pregnancies. Studies were identified through a search of several electronic databases and manual searches of bibliographies of known primary and review articles. Studies were selected if they undertook transvaginal sonographic cervical assessment among an asymptomatic population of twin pregnancies with known gestational age at delivery. Of 54 potentially relevant studies, 17 met inclusion criteria for meta-analysis. These studies included a total of 3,512 women mostly of whom were screened between 20-24 weeks′ gestation. Data were extracted for the studies′ characteristics and quality and used to form 2 X 2 contingency tables for various cervical length measurements with birth prior to 32, 34, and 37 weeks′ gestation as the reference standards. Data were stratified according to gestational age at testing, cervical length threshold, and the various reference standards, and were pooled to produce summary estimates of several measures of diagnostic accuracy, including likelihood ratios and area under the summary ROC (sROC) curve. The most commonly reported sub-group used a cervical length threshold of 25 mm with spontaneous preterm delivery prior to 34 weeks′ as the reference standard. The summary LR+ for this group was 5.12 (95% CI 4.28 – 6.13) with corresponding LR- of 0.62 (95% CI 0.57 – 0.68). The area under the curve for the sROC was 0.907 (95% CI 0.840 – 0.974). The predictive accuracy was lower for the remaining cut-offs assessed and reference standards utilized. This systematic review shows that a short cervical length (25 mm) as noted by transvaginal cervical sonography may be useful for the identification of women with twin pregnancies who are at increased risk for spontaneous preterm delivery.