Monochorionic Twins Have One Placenta
Monochorionic twins share a single placenta with vascular connections between the two fetal circulations. 1
Key Anatomical Features
Monochorionic twins possess one common placental mass, regardless of whether they are diamniotic (two amniotic sacs) or monoamniotic (one amniotic sac). 1
Vascular anastomoses (connections) are present in over 95% of monochorionic twins, linking the two fetal circulations through the shared placenta. 1
The single placenta contains intertwin vascular communications that allow blood transfer between the twins, which is the anatomical basis for complications unique to monochorionic pregnancies. 2, 3
Clinical Significance of the Single Placenta
The shared placental circulation creates substantially higher risks compared to dichorionic twins:
Perinatal mortality is more than 2-fold higher in monochorionic twins (7.6% fetal demise rate) compared to dichorionic twins (1.6% fetal demise rate). 1
The single placenta with vascular connections predisposes to twin-twin transfusion syndrome (TTTS) in 8-12% of cases, twin anemia-polycythemia sequence (TAPS), and selective fetal growth restriction. 1
Unequal placental sharing and imbalanced blood flow through the anastomoses cause these life-threatening complications. 1, 4
Distinguishing from Dichorionic Twins
Dichorionic twins have two separate placentas (or two distinct placental masses that may be fused), each serving one twin independently. 1
Ultrasound identification of a "T" sign indicates monochorionic placentation (single placenta), while a "lambda" or "twin peak" sign indicates dichorionic placentation (two placentas). 1
Common Pitfall
Do not confuse monochorionic with monoamniotic—monochorionic refers specifically to the number of placentas (one), while monoamniotic refers to the number of amniotic sacs (one). Most monochorionic twins are diamniotic (two sacs, one placenta), occurring in about two-thirds of monozygotic twins. 1