Evaluation of the twin peak or lambda sign in determining chorionicity in multiple pregnancy.

Abstract

To assess prospectively the diagnostic capabilities of ultrasound assessment, using the twin peak or lambda sign, in determining chorionicity in multiple pregnancy. Fifty-five cases of multiple pregnancy were assessed. Real-time ultrasound scans of the origin of the inter-twin membrane for the presence or absence of the twin peak/lambda sign were performed and permanent images recorded. Chorionicity was determined by placental pathologic analysis. Presence or absence of the twin peak/lambda sign as determined by real-time ultrasound correctly predicted chorionicity in 34 of 36 dichorionic and seven of eight monochorionic twin pregnancies: sensitivity for dichorionicity 94%, specificity 88%, positive predictive value 97%, and negative predictive value 78%. Real-time assessment was superior to analysis of permanent films. Ultrasound assessment of chorionicity using the twin peak/lambda sign has high sensitivity and specificity, but accuracy may not be sufficient to guide clinical management in all cases.