Challenges in Determining Chorionicity and Diagnosing Selective Fetal Growth Restriction in Multifetal Pregnancies with Suspected Twin-Twin Transfusion Syndrome: A Case Report.

Introduction: Chorionicity is a key determinant of perinatal outcomes in twin pregnancies, with monochorionic gestations carrying greater risks due to complications such as twin-to-twin transfusion syndrome (TTTS) and selective fetal growth restriction (sFGR). Although first-trimester ultrasound can determine chorionicity with near-perfect accuracy, assessment becomes less reliable in late gestation, increasing the risk of misclassification. Such diagnostic errors may lead to inappropriate management strategies, particularly when distinguishing between TTTS and sFGR, which require different clinical approaches. Case Presentation: A 38-year-old woman underwent routine antenatal assessment at 34-35 weeks' gestation, where ultrasound suggested a monochorionic diamniotic pregnancy with suspected TTTS based on the presence of T-sign and discordant amniotic fluid volumes. The diagnostic turning point occurred at delivery, when postnatal evaluation revealed a dichorionic diamniotic placenta, clarifying that the findings were consistent with sFGR rather than TTTS. The first neonate weighed 2610 g with a normally sized placenta (20 × 20×2 cm), while the second weighed 510 g with a markedly smaller placenta (10 × 10×1.5 cm). According to the Delphi criteria, the diagnosis of sFGR was confirmed, as the smaller twin had an estimated fetal weight below the 10th percentile and intertwin weight discordance exceeded 25%. Conclusion: This case illustrates how late-gestation assessment of chorionicity can lead to diagnostic misclassification, resulting in management strategies that may not align with the underlying pathology. Early and regular ultrasound surveillance-ideally beginning in the first trimester and continued every 2-4 weeks-remains essential for accurate diagnosis and appropriate monitoring of complications such as TTTS and sFGR.