Twin-to-Twin Transfusion Syndrome

Twin-Twin Transfusion Syndrome (TTTS) is a condition that can affect twin gestations that share 1 placenta. This disorder highlights the importance of determining the chorionicity (number of placentas) and amnionic (number of amniotic sacs) for all twin gestations, influencing management. Twin gestations can be dichorionic diamniotic (2 placentas, and 2 amniotic sacs), monochorionic diamniotic (MCDA, 1 placenta, and 2 amniotic sacs), or monochorionic monoamniotic (MCMA, 1 placenta, and 1 amniotic sac).  In the most accepted model of monozygotic twinning, the number of placentas and amniotic sacs depends upon when the splitting of the zygote occurs; dichorionic diamniotic twins result when splitting occurs between days 1 to 3, MCDA twins result when splitting occurs between days 3 to 8, MCMA twins result when splitting occurs between days 8 to 13, and conjoined twins result when splitting occurs on or after day 13. Ultrasound findings that help distinguish dichorionic and monochorionic twin gestations include a “lambda sign” in DC and a “T sign” in MC twin gestations. The “T sign” is created by the thin dividing membrane between 2 amniotic sacs when there is only 1 placenta supporting both gestations (monochorionic). This requires an absence of the “lambda” or “twin peak” sign, which is seen when each gestation is supported by its placenta (dichorionic). The greatest risk of developing TTTS is in monochorionicity, which is more common in MCDA twins than MCMA twin gestations. To diagnose TTTS prenatally, an ultrasound must show a single placenta, 1 twin with oligohydramnios and the other with polyhydramnios. Oligohydramnios is typically defined as a maximal vertical pocket of < 2 cm. In comparison, polyhydramnios is typically defined as a maximal vertical pocket of > 8 cm. Growth discordance and intrauterine growth restriction can occur but are not required to diagnose TTTS. In monoamniotic twin gestations, findings consist of polyhydramnios and fetal bladder differences.