Trimester-specific accuracy of ultrasound in diagnosing abnormal umbilical cord insertion in monochorionic twins.

INTRODUCTION: Abnormal umbilical cord insertion may influence the risk of adverse outcomes in monochorionic twin pregnancies. This study aimed to evaluate the diagnostic accuracy of prenatal ultrasound in identifying different types of umbilical cord insertion across pregnancy trimesters. METHODS: This prospective monocentric study was conducted at Fondazione Policlinico Universitario Agostino Gemelli IRCCS, Rome, between 2021 and 2025. Monochorionic twin pregnancies with prenatal ultrasound assessment, obstetric outcomes, and postpartum placental examination were included. Umbilical cord insertion was assessed during each trimester, using placental examination as the reference standard. Sonographic classification included eccentric, marginal, and velamentous insertions, and proximate cord insertion (pPCI), defined as insertions ≤4 cm apart. Diagnostic performance was evaluated through sensitivity, specificity, predictive values, likelihood ratios, and accuracy. RESULTS: Among 184 monochorionic twin pregnancies, placental examination identified eccentric, marginal, and velamentous insertions in 53.0%, 35.0%, and 12.0% of cases, respectively. pPCI occurred in 9.3% (17/182). Ultrasound showed a sensitivity of 0.79, specificity of 0.93, and overall accuracy of 0.87 (95% CI 0.83-0.90). Accuracy improved from 60.0% in the first trimester to 80.9% and 77.5% in the second and third trimesters. Detection of pPCI showed 99% accuracy. Posterior placental location was associated with reduced diagnostic accuracy (OR 0.24; 95% CI 0.12-0.43; p < 0.001). DISCUSSION: Prenatal ultrasound reliably identifies umbilical cord insertion abnormalities in monochorionic twin pregnancies, with optimal performance especially in the second trimester, supporting its routine antenatal assessment.