Monochorionic Twins Have ONE Placenta, Not Two
No, monochorionic twins do not have 2 placentas—they share a single placental mass. This is the defining characteristic that distinguishes them from dichorionic twins and is the anatomical basis for their unique complications 1.
Understanding Placentation in Monochorionic Twins
Monochorionic twins possess a single shared placenta with vascular anastomoses connecting the two fetal circulations 1. This occurs when monozygotic (identical) twins split between 4-8 days after fertilization, resulting in monochorionic-diamniotic (MCDA) twins with one placental mass and two amniotic sacs 1.
Key Anatomical Features:
- Single placental mass is present in over 95% of monochorionic twins, with intertwin vascular anastomoses connecting the circulations 1
- The shared placenta may appear as one continuous mass or, rarely (in approximately 3% of cases), as a bipartite placenta with two separate lobes connected only by membranes and vessels—but this is still considered ONE monochorionic placenta 2, 3
- Ultrasound shows a "T" sign where the intertwin membrane meets the placenta perpendicularly, indicating absence of chorionic tissue between the membrane layers (sensitivity nearly 100%, specificity 98%) 1
Clinical Significance of Single Placenta
The single shared placenta is directly responsible for the significantly elevated risks in monochorionic twins:
- Perinatal mortality is >2-fold higher in monochorionic versus dichorionic twins (7.6% vs 1.6% fetal demise rate) 1
- Twin-twin transfusion syndrome (TTTS) occurs in 8-12% of monochorionic twins due to imbalanced blood flow through placental anastomoses 1
- Overall mortality rate is approximately 10% for monochorionic-diamniotic pregnancies 1
Important Caveat: The Bipartite Placenta Exception
While monochorionic twins have ONE placenta, this can occasionally appear as two separate masses on ultrasound:
- Bipartite monochorionic placentas occur in ~3% of cases, where two placental lobes are connected only by membranes and vascular anastomoses 2
- These are still monochorionic (confirmed by histology showing no chorionic tissue in the dividing membrane) and still have vascular connections 2
- Bipartite placentas are associated with higher rates of TTTS and selective growth restriction and may complicate surgical interventions 3
The presence of two separate placental masses does NOT automatically mean dichorionic twins—the "T" sign and membrane histology are definitive 2.
Contrast with Dichorionic Twins
Dichorionic twins (whether dizygotic or monozygotic with early splitting <4 days) have: