Monochorionic monozygotic twin pregnancy complicated with twin-twin transfusion syndrome presenting with an obvious lambda sign in the first trimester.

In this report, we present a case of twin-twin transfusion syndrome in the presence of the thick dividing wall of each sac in dichorionic twins. The dichorionic diagnosis was based on the presence of the lambda sign at the first-trimester ultrasound evaluation. In addition to fetoscopic and pathological investigation, DNA typing confirmed that the twin set was monochorionic and monozygotic. This case illustrates that although extremely uncommon, the presence of lambda sign does not exclude monochorionic pregnancy.