The fetoscopic laser therapy  is currently recognized as the first-line treatment for stage I to IV TTTS diagnosed before 26 weeks of gestation, resulting in better survival and neurological outcomes of fetuses. However, because laser therapy has to coagulate the placental anastomotic vessels between the two fetuses in TTTS, the decreased placental perfusion to the donor twin with IUGR may cause a higher fetal demise rate than in the TTTS donor without IUGR . Furthermore, the procedure of laser therapy is not free of complications. In our series, 11.4% of treated cases experienced premature rupture of membranes (PROMs) within 3 weeks, and 2.2% of treated cases developed chorioamnionitis needing termination . So in the TTTS twins where the donor has IUGR and abnormal UA Doppler, the poorer fetal outcomes should be discussed with the pregnant woman. In laser therapy for TTTS with sIUGR, it is important to preserve the blood vessels supplying to the donor twin as much as possible. In the sequential selective laser photocoagulation of communicating vessels (SQLPCV) technique, occlusion of the vascular communications is done in a particular order, resulting in a transient intra-operative net transfusion to the donor twin and facilitating a better hemodynamic equilibrium . SQLPCV may be the laser strategy of choice for managing the intertwin anastomoses for TTTS with sIUGR.
After laser coagulation of the inter-twin anastomoses in the placenta of TTTS twins, the placenta became functionally dichorionic, resulting in the resolution of the polyhydramnios-oligohydramnios sequence. Group 1 twins are very similar to monozygotic, dichorionic twins with sIUGR, in which the two fetuses share identical DNA content without twin-twin vascular anastomoses since they have two separate placentas, but one fetus develops IUGR. In group 1 twins, placental mtDNA contents were 1.57-fold higher in the placental territory of the sIUGR twin than that of the larger twin (Fig. 1). Our findings were similar to previous findings in singleton pregnancies, in which placental mtDNA contents are higher in pregnancies with IUGR than in normal pregnancies .
Our findings that placental mtDNA fold changes were significantly higher in group 1 (those without patent vascular anastomoses) than in the group 3 twin pregnancies (those with patent vascular anastomoses) indicate that blood shunting may temper the increase of the placental mtDNA content in the sIUGR twin. MtDNA increases have been associated with hypoxia in the placentas of IUGR . Our results further suggest that the rescue perfusion from the AGA- to sIUGR-fetuses determines the placental hypoxic status in the sIUGR twins between these two groups.
One potential etiology of sIUGR in MC twins is its low placental share . Our results (Fig. 2) of similar placental discordance in the TTTS with sIUGR (group 1) and the no TTTS twins with sIUGR (group 3) provided evidence against the hypothesis that the different degrees of low placental share in the sIUGR twins led to different placental mtDNA contents between these two groups.
We have previously reported that the fetal / placenta weight (F/P) ratio is increased in MC twins with sIUGR  when rescue perfusion from the larger placental territory to the sIUGR one through vascular anastomoses works to promote growth of the sIUGR twin. By this compensatory mechanism, the sIUGR twin in each pair of shunt-patent twins (group 3) gets more perfusion than its small placental territory would usually supply, resulting in the alleviation of hypoxic stressors and a decrease of mtDNA content in the sIUGR placental territory. On the other hand, laser therapy disrupts the vascular shunt between the TTTS twins and terminates the aforementioned rescue perfusion, resulting in hypoxia and, thus, increased mtDNA contents in the placental territory of the sIUGR twin. The placental mtDNA fold change between the smaller and larger twins was not different between group 2 and 4 twin pregnancies (Fig. 1). This finding shows that, in MC twins without sIUGR but with TTTS (group 2), blocking the vascular anastomoses to treat TTTS did not cause hypoxia in the placental territory of either twin and did not increase the placental mtDNA content.
It is important to point out that the fetal blood of the sIUGR twin in MC twins contains significantly higher mtDNA , representing a higher state of stress in the sIUGR fetus. However, the hypoxic status of the placental compartment is quite different. In the presence of vascular anastomoses between the MC twins without TTTS (which do not require laser therapy), the presence of sIUGR (group 3) triggers a rescue shunt that effectively relieves hypoxia resulting in decreased placental mtDNA fold changes.
Limitations of this study do exist. First, the case number in the TTTS with sIUGR in this study is small (n = 7). After laser therapy, the survival rate of both twins in MC pregnancies complicated by TTTS in our center was around 55% (55/100), and the survival rate was further decreased to 33.3% (12/36) when the TTTS twins were complicated by sIUGR in our center [21, 24]. Therefore, it was not easy to collect the 7 cases of TTTS with sIUGR with both twins surviving after laser therapy and with intact placentas after delivery. Second, we could not exclude the possibility that the increased placental mtDNA fold changes in group 1 had been caused by the insult of polyhydramnios-oligohydramnios before laser therapy, although the demographic data between groups 1 and 3 were similar (Table 1, P1). The hypothetical detrimental effects of polyhydramnios-oligohydramnios was further weakened by the findings of similar placental mtDNA fold changes between groups 2 and 4 (Fig. 1). Third, we carefully examined every placenta with visual inspection, but did not perform dye-injection study. Therefore, residual anastomoses, especially tiny ones, cannot be completely ruled out. Lastly, rinsing the placental tissues with ice-cold PBS might not completely remove the contamination of maternal blood and thereby maternal mtDNA. However, since the comparison was done in each pair of MC twins, the effect of contamination by maternal mtDNA was equally on each twin, thus being minimized. The consideration again supports the advantage of using MC twins for sIUGR studies.