Clinical meaning
Twin-to-twin transfusion syndrome (TTTS) is a serious complication of monochorionic diamniotic (MCDA) twin pregnancies, affecting approximately 10-15% of MCDA twins. It results from unbalanced blood flow through placental vascular anastomoses (arteriovenous, arterioarterial, and venovenous connections) that connect the circulations of the two fetuses sharing a single placenta. In normal MCDA pregnancies, bidirectional flow through AA anastomoses compensates for unidirectional AV flow, maintaining hemodynamic balance. TTTS develops when there is net unidirectional blood flow from the donor twin to the recipient twin through deep AV anastomoses without adequate compensatory AA anastomoses. The donor twin becomes progressively hypovolemic, leading to renal hypoperfusion, oliguria, oligohydramnios (reduced amniotic fluid — stuck twin appearance with membrane closely wrapped around the fetus), growth restriction (IUGR), and eventually hydrops and death. The recipient twin becomes hypervolemic, leading to cardiomegaly, polyuria, polyhydramnios (excess amniotic fluid causing uterine distension), high-output cardiac failure, hydrops fetalis, and potentially death from cardiac decompensation. Without treatment, TTTS has a near-100% perinatal mortality rate for severe cases. The Quintero staging system classifies severity: Stage I (polyhydramnios/oligohydramnios discordance; donor bladder visible), Stage II (donor bladder not visible — absent urine production from severe renal hypoperfusion), Stage III (critically abnormal Doppler studies — absent/reversed end-diastolic flow in umbilical artery, reversed a-wave in ductus venosus, pulsatile umbilical venous flow), Stage IV (hydrops in one or both twins), Stage V (fetal demise of one or both twins).