Background: Determination of placentation and amnionicity is important for antenatal and intrapartum management in twin gestations. We present an typical monozygotic twin placenta and underscore the sonographic features and diagnostic pitfalls that may interfere with antenatal diagnosis. Case report: A 35-year-old G4P2 with a spontaneous pregnancy applied for routine second trimester anomaly screening. Upon prenatal ultrasonography at 20 weeks, an anatomically normal, same sex twin pregnancy was detected. A thin dividing membrane and a T-sign suggestive of a monochorionic diamniotic pregnancy were visualized. Antenatal follow-up was uneventful. Cesarean delivery was performed at 37 week. Postpartum examination revealed two distinct placental discs connected with each other through membranes. Microscopic examination of the membranes supported monochorionicity. Short Tandem Repeat Profile Analysis revealed identical pattern and confirmed monozygosity. Conclusion: Two separate placentas with a T-sign in same-sex twins should raise the suspicion of monozygosity, and should be followed accordingly.