Twin pregnancies with two separate placental masses can still be monochorionic and have vascular anastomoses

Objective This study was undertaken to report the occurrence of bipartite monochorionic twin placentas. Study design Examination of 109 monochorionic placentas delivered at our institution between June 2002 and June 2005 was performed. Placental characteristics on prenatal ultrasound were studied, including single or double appearance and type of intertwin membrane-placental junction (“T” sign or lambda sign). Monochorionicity was confirmed by postnatal histologic confirmation (diamniotic intertwin membrane without chorionic tissue within the dividing septum). Bipartition was diagnosed when 2 separate placental masses attached by membranes were identified. Results Of the 109 monochorionic placentas, 3 were composed of 2 separate placental masses. Prenatal ultrasound examination showed 2 separate placental masses in each case. Monochorionicity was suspected on prenatal ultrasound because of the presence of “T” sign in 2 cases and twin-to-twin transfusion syndrome (TTTS) in another case. Microscopic examination of the dividing septum was consistent with monochorionicity in each case. Vascular anastomoses were present in 2 of the 3 placentas, and led in both cases to the development of TTTS. Conclusion Two separate placental masses in twin pregnancies are not per se dichorionic and may occur in almost 3% of monochorionic placentas