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.