title: First-trimester ultrasound determination of chorionicity in twin gestations using the lambda sign: a systematic review and meta-analysis.
abstract: OBJECTIVE
  To evaluate the accuracy of first-trimester sonographic determination of chorionicity in twin gestations using the lambda sign.
  
  
  METHODS
  Electronic databases (MEDLINE, PROSPERO, Scopus, ClinicalTrials.gov, EMBASE, Sciencedirect) were searched from their inception until April 2016. We included only study assessing the accuracy lambda sign in prediction of monochorionicity in the first trimester. Forest plots for pooled sensitivity and specificity with 95% confidence intervals (CI) were generated. In addition, symmetric summary receiver-operating characteristic curves were plotted. The area under the curve (AUC) was also computed to evaluate the overall accuracy of the diagnostic test.
  
  
  RESULTS
  Nine studies, including 2292 twins, were analysed. In all of these studies, identification of the lambda sign was used to diagnose chorionicity on real-time B-mode imaging. Twins were classified as monochorionic if there was a single placental mass in the absence of the lambda sign, and dichorionic if there was a single placental mass but the lambda sign was present or the placentas were not adjacent to each other. In all nine studies, placental histology or discordant fetal sex were used to confirm chorionicity. Pooled results from the meta-analysis showed that sensitivity of the presence of the lambda sign in the prediction of dichorionicity was 99% (95% CI 98-100%), and specificity was 95% (95% CI 92-97%). Pooled sensitivity of the absence of the lambda sign in the prediction of monochorionicity was 96% (95% CI 92-98%) and pooled specificity was 99% (95% CI 98-99%). The AUC for diagnostic accuracy was 0.99, and suggested very high diagnostic accuracy.
  
  
  CONCLUSIONS
  The lambda sign predicts chorionicity with a high degree of accuracy before 14 weeks of gestation. Presence of the lambda sign indicates dichorionicity, and absence of the lambda sign indicates monochorionicity. All hospitals should encourage departments providing ultrasound services to determine chorionicity when examining women with twin pregnancies in the first trimester. As determination of chorionicity is most accurate before 14 weeks when the amnion and chorion have not yet fused, the first-trimester scan in twin pregnancy is paramount.
authors.affiliation: Department of Neuroscience, Reproductive Sciences and Dentistry, School of Medicine, University of Naples Federico II, Naples, Italy.
authors.forename: G M
authors.initials: GM
authors.lastname: Maruotti
authors.affiliation: Department of Neuroscience, Reproductive Sciences and Dentistry, School of Medicine, University of Naples Federico II, Naples, Italy.
authors.forename: G
authors.initials: G
authors.lastname: Saccone
authors.affiliation: Department of Neuroscience, Reproductive Sciences and Dentistry, School of Medicine, University of Naples Federico II, Naples, Italy.
authors.forename: M
authors.initials: M
authors.lastname: Morlando
authors.affiliation: Department of Neuroscience, Reproductive Sciences and Dentistry, School of Medicine, University of Naples Federico II, Naples, Italy. Electronic address: martinel@unina.it.
authors.forename: P
authors.initials: P
authors.lastname: Martinelli
mesh_terms: D002823:Chorion / Q000000981:diagnostic imaging*; D005260:Female; D006801:Humans; D010920:Placenta / Q000000981:diagnostic imaging*; D011247:Pregnancy; D011261:Pregnancy Trimester, First; D059285:Pregnancy, Twin; D012680:Sensitivity and Specificity; D014427:Twins; D016216:Ultrasonography, Prenatal / Q000379:methods*
keywords: First trimester; Multiple gestations; Twin–twin transfusion syndrome; Ultrasound