Twin to twin transfusion syndrome

Twin pregnancies represent 1.2 % of spontaneously conceived pregnancies. Twins can either be dizygotic, after fertilization of two ova, or monozygotic, arising from the fertilization of a single ovum [1]. Only monozygotic (identical twin) can be monochorionic. The development of a monochorionic multiple gestation pregnancy is based on the day of embryonic splitting. If the embryo splits before day 3 after the fertilization, then two independent fetuses with separate placentas (dichorionic) will result. However, splitting after day 4 will result in a monochorionic diamniotic gestation. Division after day 8 results in a monochorionic monoamniotic gestation [1]. Owing to vascular connections within a single placenta, monochorionic gestations present distinctive prenatal management challenges, because an unbalanced hemodynamic exchange between fetuses could arise. This condition is called “twin to twin transfusion syndrome” (TTTS), and it is a serious progressive fetal pathology that occurs in 10–20 % of monochorionic diamniotic twin pregnancies [2]. In TTTS, disproportionate intertwin transfusion, via placental anastomoses, causes circulatory depletion in one twin (donor) and overload in the other (recipient) [2]. These anastomoses can be artery to artery (AA), vein to vein (VV), or artery to vein (AV) [3].