Interpretation
TTTS complicates only a small proportion of monochorionic diamniotic (MC DA) twin pregnancies even though intertwin placental anastomoses are demonstrated in the vast majority of MC DA twin pregnancies with or without TTTS19-23. Thus, the presence of placental anastomoses is necessary but not sufficient for the net transfer of blood from the donor to the recipient twin in TTTS. Unequal distribution of the placental mass may also contribute to the pathogenesis of TTTS since the estimated fetal weight of the donor twin is typically smaller than the recipient twin; however, if we use selective fetal growth restriction as a proxy of unequal placental sharing, sFGR is seen in only a small proportion of TTTS cases as demonstrated by the 21% prevalence of sFGR in our study. The smaller placental mass allocated to the donor twin should have a smaller placental bed vasculature and higher downstream vascular resistance than the placental mass allocated to the recipient twin. However, this may not always be the case because we observed that impedance to blood flow in the umbilical arteries was higher in the recipient twin than in the donor twin in 43.3% of the TTTS cases requiring laser surgery. Thus, it is possible that the allocation of the placental bed vasculature responsible for downstream umbilical artery resistance may not necessarily correspond to the allocation of placental mass in all TTTS cases. Our observations that small intertwin differences in UA impedance to blood flow are associated with increased double twin survival to 30 days of life in TTTS cases following laser surgery suggest that intertwin differences in UA PI may provide novel metrics to evaluate the severity and disease progression in TTTS9.
The observation that small intertwin DUAPI (<0.4) supersedes the Quintero classification or any of its sonographic criteria in the prediction of double twin survival when the analysis is adjusted for gestational age at delivery, sFGR and other important confounders is noteworthy. This was observed in the multivariable regression analysis of the whole study cohort, as well as in women with TTTS Quintero stage I or II combined as well as in those with TTTS Quintero stage III or IV combined. This is probably because intertwin DUAPI may better reflect the underlying mechanisms or disease in TTTS compared to the sonographic criteria used in the Quintero classification, which relies on amniotic fluid volume in both twins, significant fetal Doppler anomalies and the presence or absence of fetal hydrops, which tend to normalize following laser surgery. Intertwin differences in UA impedance to blood flow may also improve following laser surgery, but these intertwin differences may better reflect the severity of the disease processes leading into TTTS in view of their association with double infant survival.
Since the Quintero classification is widely used to evaluate TTTS cases, we propose using an intertwin DUAPI of <0.4 to further stratify the Quintero staging system in order to evaluate the possibility of double infant survival following laser surgery. In the sub analysis restricted to individual Quintero stages, women with a DUAPI<0.4 had higher double infant survival than those with DUAPI ≥0.4. These differences were significant in Quintero stages I and III, but not in Quintero stages II and IV as individual groups. This is likely because our study was underpowered to perform these sub-analyses.