Monochorionic–Monoamniotic Twins

Twin–Twin Transfusion Syndrome in Monochorionic–Monoamniotic Twins

The incidence of monochorionic–monoamniotic twins ranges between 1% and 3% of twin births.18,30 Aside from few case reports,31 TTTS has rarely been documented in monochorionic– monoamniotic twins, despite the presence of extensive vascular anastomoses similar to those found in monochorionic–diamniotic twins.

Benirschke and Driscoll32 attribute the absence of the syndrome in the monoamniotic twin category to either a lack of the arteriovenous anastomoses between twins or a more extensive anastomoses connection that may balance the net volume between twins. Similarly, Umur et al33 have found a significantly higher arterioarterial anastomoses in monochorionic–monoamniotic twin placentas compared to monochorionic– diamniotic twin placentae (100% vs 80%), and significantly lower arteriovenous anastomoses (50% vs 83%).


This may imply that placentas of monoamniotic twins are more likely to have extensive and balanced anastomosis than those of diamniotic twins. By correlating the percentage of the TTTS in monochorionic–diamniotic twins with arterioarterial anastomoses, the authors were able to draw a parallel to extrapolate an incidence rate of developing TTTS in monochorionic–monoamniotic twins to be 3.8% (2.6 times lower than the incidence in monochorionic– diamniotic twins). Suzuki et al31 (hospital-based cohort) had estimated 9.6% of their monochorionic–monoamniotic twins to have TTTS. Obstetrical criteria such as amniotic fluid discordance in monoamniotic twins is not useful for the diagnosis of TTTS. The higher mortality associated with cord entanglement in monoamniotic twins may affect the incidence of TTTS in liveborn twins. Lutfi showed no TTTS in 48 monochorionic–monamniotic twin pregnancies.18