The diagnosis of twin fetus transfusion syndrome was based on ultrasound diagnostic criteria during pregnancy, single chorionic vesicle twin fetuses appeared in the sequence of too much amniotic fluid C too little, that is, before 20 weeks to meet the maximum depth of amniotic fluid (Maximal vertical pocket, MVP) > 8cm in one fetus (blood recipient), while the MVP of the other fetus (blood donor) < 2cm, after 20 weeks to meet the MVP of one fetus (blood recipient) > 10cm, while the MVP of the other fetus (blood donor) < 2cm. Stage I: the bladder of the donor is still visible; Stage II: the bladder of the donor is no longer visible; Stage III: abnormal blood flow in either fetus; Stage IV: edema in the recipient; Stage V: death of either or all fetuses. Twin transfusion syndrome is a name that has been used to reflect the pathogenesis of TTTS to some extent, but numerous studies have suggested that there is no significant difference in hemoglobin concentration between the donor and recipient, so in order to distinguish it from the Twin anemia polycythemia sequence (TAPS), which will be mentioned below. Therefore, it has been suggested to change it to the twin oligopolyhydramnios sequence (TOPS). The pathophysiological mechanism of twin transfusion syndrome (TTTS) is complicated by TTTS in approximately 10% of single chorionic twin pregnancies, usually between 16 and 26 weeks of gestation. The etiology of TTTS is still unknown due to the lack of animal models of TTTS and the somewhat restricted collection of fetal blood samples during pregnancy. The presence of interplacental vascular anastomotic branches is the anatomical basis for inter-twin transfusion, and secondary fetal blood volume changes are the main pathophysiological changes. In addition to meeting its own needs, the blood donor has to transfuse blood to the recipient, which may lead to anemia, intrauterine growth restriction and low amniotic fluid. After birth, they show anemia, low weight, insufficient circulating blood volume, hypotension, growth retardation and even death by shock. On the contrary, the recipient is constantly receiving blood transfusions from the donor, and is overloaded with circulating blood, showing signs of polycythemia, high blood viscosity, high blood pressure, cardiac hypertrophy, subcutaneous edema, excessive amniotic fluid, a redder skin tone after birth, or in severe cases, dark red or purple skin, relatively heavy weight, and prone to complications such as congestive heart failure, hyperbilirubinemia and bilirubin encephalopathy. In addition, a large number of studies suggest that although TTTS manifests as differences in amniotic fluid between the two fetuses, there is no difference in postnatal hemoglobin concentration between the two fetuses, suggesting that there may be endocrine factors affecting fluid balance between the two fetuses. Anastomotic vessels and twin-fetus transfusion syndrome The currently used fetal microscopic in vivo observation of the placental surface structure and postnatal superficial placental vascular perfusion suggest that the presence of at least one AV anastomotic vessel on the superficial surface of the placenta is a prerequisite for TTTS complicating a single-chorionic twin-fetus pregnancy. de Taylor et al. found that TTTS occurred in only 15% of cases with A-A vessel anastomosis and in 61% of cases with the opposite. de Villiers et al. found only 37% of TTTS placentas with A-A anastomosis and 91% of non-TTTS placentas with A-A anastomosis, and Lopriore et al. found only 47% of TTTS placentas with A-A anastomosis and 96% of non-TTTS placentas with A-A anastomosis, all suggesting that the presence of AA anastomotic vessels may be a protective factor against TTTS in single-chorionic twin pregnancies. Umur et al. demonstrated through computerized mathematical modeling simulations of TTTS that AA anastomotic vessels have less resistance to blood flow than reverse AV anastomotic vessels that require blood exchange through the chorionic villi, and therefore AA anastomotic vessels can more effectively replenish the imbalanced blood volume of the donor compared with reverse AV anastomotic vessels, which supports to some extent that AA anastomotic vessels are more effective in compensating for the imbalanced blood volume of the donor. anastomotic vessels as a protective factor against TTTS in monochorionic twins. In addition, Lopriore et al. suggested that the median AA anastomotic vessel diameter sum was 0.6 mm and 1.7 mm in the TTTS and non-TTTS groups, respectively, with a significant difference, suggesting that AA anastomotic vessel diameter may also influence the development of TTTS. The effect of VV anastomotic vessels in TTTS has been little studied, both because of the small incidence of VV anastomotic vessels in the superficial surface of the placenta in monochorionic twin pregnancies and because of the mixed factor of the presence of AA anastomotic vessels.Lopriore et al. 2014 reported 630 monochorionic twin pregnancies with the exclusion of cases with AA anastomotic vessels in the superficial surface of the placenta, leaving a total of 30 cases TTTS and 41 non-TTTS cases, comparing the difference in VV anastomotic vessels between the two groups, the study showed that the incidence of VV anastomotic vessels was significantly higher in the TTTS group than in the non-TTTS group (37% vs 7%, P < 0.01), suggesting that VV anastomotic vessels may increase the risk of developing TTTS in the absence of the effect of AA anastomotic vessels (OR 7.3, 95% CI 1.8- 37.1, P < 0.01). Some authors have performed superficial vascular perfusion of placentas that have undergone fetoscopic laser treatment in an attempt to analyze whether there is a correlation between fetal prognosis and residual vessels after FLOC surgery. Lewi et al. reported the results of placental studies in 50 patients with TTTS after FLOC, in which residual anastomotic vessels were present in 16 patients, including 7 cases of twin fetal death and 9 cases of twin fetal survival. 7 cases of twin fetal death had only AV vascular anastomoses >1 mm in diameter in the postoperative placenta, 6 cases of twin fetal survival had only AV vascular anastomoses <1 mm in diameter in the postoperative placenta, and 2 cases had AV anastomoses with concomitant large AA or VV anastomotic vessels, suggesting a better perinatal outcome when no residual vessels or large AA residuals are present after FLOC. Lopriore et al. reported the results of a placenta study in 77 patients with TTTS after FLOC, and similarly found that the presence of residual anastomotic vessels in the placenta of TTTS patients after FLOC amounted to 33%, and there was no significant difference in perinatal outcome between the two compared with patients without residual anastomotic vessels after FLOC, considering that the presence of residual anastomotic vessels in the placenta may be due to the presence of protective AA and VV anastomotic vessels. Placental share, umbilical cord attachment position and twin-fetus transfusion syndrome Most scholars currently believe that structural differences in anastomotic vessels are not sufficient to explain the pathogenesis of TTTS, Bruner et al. found an increased incidence of placental share inequality in TTTS patients in a small uncontrolled study as early as 1998, and De Paepe et al. reported the same results in 2005, finding that in TTTS patients De Paepe et al. reported the same results in 2005, finding a significantly higher incidence of placental share imbalance in TTTS patients, but also found that placental share imbalance was more often combined with the presence of umbilical cord sail attachment, suggesting that placental share imbalance may lead to the development of TTTS due to hemodynamic imbalance between the two fetuses. Similarly, some early studies in small samples suggested an increased incidence of sail-like attachment of the umbilical cord in TTTS placentas, suggesting that there may be a vicious cycle of uteroplacental function limitation due to umbilical cord sail-like attachment and subsequent blood volume deficiency, which may cause the development of TTTS. Fries et al. found that the incidence of sail-shaped placenta was as high as 63.6% in TTTS and only 18.5% in non-TTTS. Therefore, Fries et al. suggested that the umbilical cord with sail-shaped attachment vasculature is not protected by warton glue and is prone to compression when the fetal position is changed, which may lead to a decrease in fetal blood flow on that side, causing an imbalance in blood flow pressure between the two fetuses and subsequent TTTS, suggesting that umbilical cord with sail-shaped attachment may be a risk factor for the development of TTTS. It was suggested that umbilical cord sail attachment might be a risk factor for the development of TTTS. However, most of the later studies suggested that the uneven share of placenta is often combined with umbilical cord sail attachment or marginal attachment. In contrast, Quintero et al. in 2005, Lopriore et al. in 2007, and Costa-Castro et al. in 2013 proposed that there were no significant differences in the incidence of marginal or sail-like attachment of the umbilical cord and uneven placental share between TTTS and non-TTTS patients in large sample data, and all three authors concluded that uneven placental share and sail-like attachment of the umbilical cord were not risk factors for the development of TTTS. All three authors concluded that placenta share irregularity and umbilical cord sail attachment were not risk factors for the development of TTTS. Therefore, the influence of placenta share and umbilical cord attachment position on the development of TTTS is still controversial. The clinical findings of selective intrauterine growth restriction with increasing gestational weeks or both fetuses after FLOC are not available, and the pathogenesis is unclear. The author believes that before FLOC surgery, there is anastomotic vascular traffic between the two fetuses, which can compensate to some extent for the growth of small placental share fetuses, but after FLOC surgery blocks the anastomotic vascular traffic between the two fetuses, the small placental share loses its original blood flow compensation, and with the increase of gestational weeks, slow growth or even intrauterine death occurs gradually. Therefore, the placenta share of TTTS patients may have an impact on the prognosis of TTTS after FLOC surgery. Treatment of twin fetus transfusion syndrome and placental characteristics For the treatment of TTTS, the commonly used methods are amniocentesis amniotic fluid reduction, septoplasty, and fetoscopic laser coagulation placental anastomosis vascularization (FLOC). Kilby et al. suggested that the prognosis of TTTS patients is influenced by whether the bladder is visible after amniotic fluid reduction. If the bladder of the donor is visible on ultrasonography 24 hours after surgery, it suggests that TTTS patients have a better response to amniotic fluid reduction, presumably there may be more A-A anastomotic vessels on the superficial surface of the placenta in these patients, and after the reduction of amniotic fluid, the pressure in the amniotic cavity changes, which increases the compensatory blood flow from the recipient to the donor, thus improving the symptoms of TTTS to some extent; conversely, for TTTS patients with a poor response to amniotic fluid reduction A multicenter study published by Crombleholme et al. in 2007 suggested that amniotic fluid reduction appeared to be no worse than FLOC in improving neonatal survival, but the incidence of distant neonatal neurological sequelae was significantly higher than after FLOC. in 2008 Rossi et al. performed a meta-analysis showing that compared to FLOC surgery, amniotic fluid reduction for TTTS was worse in terms of neonatal survival and long-term neurological sequelae. The main reason for this is that amniotic fluid reduction does not change the structural characteristics of the superficial anastomotic vessels of the placenta and treats the symptoms rather than the root cause. The septal stoma is rarely used in clinical practice. This method involves the use of an ultrasound-guided amniocentesis needle to puncture the septum between the two fetuses, thereby artificially creating a single amniotic sac in a twin pregnancy in an attempt to alleviate the imbalance in amniotic fluid and amniotic cavity pressure between the two fetuses. However, another characteristic of the placenta, the distance between the attachment points of the umbilical cord, plays a very important role in the postoperative period, and a closer distance between the attachment points of the umbilical cord between the two fetuses will increase the risk of intrauterine fetal death of the twin fetuses due to cord entanglement. Fetoscopic laser coagulation of placental anastomotic vessels has created a great sensation since its introduction and is now the first-line treatment of choice for TTTS. The current indications for fetoscopic treatment of TTTS are for cases with Quintero stage II and above, and it has been proposed that FLOC surgery can also be aggressively used for patients with TTTS stage I combined with more severe involvement of the recipient's cardiac function. There is an ongoing multicenter international study on the need for FLOC surgical intervention in stage I. FLOC surgery for TTTS focuses on the anastomotic vessels superficial to the placenta. In the early 1990s, the main approach was to coagulate all the vessels traveling beneath the septum along the septum between the two fetuses under fetoscopy, called non-selective laser coagulation of communicating vessels. However, the position of the septum between the two fetuses does not represent the location of the placental area segmentation, and this approach cuts off some of the anastomotic vessels that are protective of the survival of the donor and those that travel below the septum but are not involved in the pathogenesis of TTTS, increasing the postoperative mortality of the donor. In the late 1990s, Quintero et al. proposed that different types of anastomotic vessels should be identified and selectively coagulated under fetoscopy, thus proposing selective laser photocoagulation of communicating vessels (S-LPCV). LPCV), which showed a significant decrease in mortality in both fetuses after the procedure. Subsequently, Quintero et al. proposed sequential SLPCV (SQ-LPCV), which involves coagulation of the superficial A-V anastomotic vessels of the placenta followed by coagulation of the A-A and V-V anastomotic vessels, and showed a significant decrease in donor mortality after SQ-LPCV compared with S-LPCV. However, the rate of residual postoperative anastomotic vessels is still high for the above three FLOC surgical approaches, which leads to the possibility of complex complications such as recurrence of postoperative TTTS, occurrence of TAPS or reverse TTTS. To address the problem of residual postoperative anastomotic vessels, a fourth coagulation of anastomotic vessels has been proposed called the Solomon style, which involves sequential coagulation of the superficial placental A-V, A-A, and V-V anastomotic vessels followed by laser connection of all coagulation points to form an equatorial line on the superficial placental chorionic plate to functionally divide the placenta into two parts, aiming to reduce the incidence of residual postoperative vessels. In 2014, Lewi et al. published a European multicenter study showing that the Solomon style significantly reduced the incidence of postoperative TTTS recurrence and TAPS, and significantly reduced fetal survival in patients with TTTS after FLOC.