The definition of sIUGR, as the name implies, is that one of the twin fetuses is growth restricted and the fetal weight is estimated by ultrasound to be below the 10th percentile of the corresponding gestational week, which is less than gestational age. The diagnostic criteria for sIUGR are still controversial. In the past, a weight difference between the two fetuses (estimated weight of the larger fetus – estimated weight of the smaller fetus)/estimated weight of the larger fetus) greater than 25% was used as the diagnostic criterion, but considering that two fetuses with a large weight difference do not necessarily have fetal growth restriction, the difference in weight between the two fetuses greater than 25% is referred to as inconsistent fetal growth. However, considering that a significant difference in weight between two fetuses is not necessarily associated with fetal growth restriction, a difference in weight between two fetuses greater than 25% is referred to as dichotomous growth. The more widely used diagnostic criterion for sIUGR is a monochorionic twin fetus in which the estimated ultrasound weight of one fetus is less than the 10th percentile of the corresponding gestational week. This definition may include cases in which both fetuses have a weight less than the 10th percentile of the corresponding gestational week and cases in which one fetus has a weight less than the 10th percentile and the difference in weight between the two fetuses is small, but since more than 95% of monochorionic twins that meet this definition actually have significant weight discrepancies between the two fetuses, and since this definition is simple and easy to apply, it has been used in clinical and research studies both at home and abroad. In recent years, scholars have gradually recognized the importance of sIUGR, and a large amount of literature suggests that sIUGR is closely related to intrauterine fetal death and neonatal neurological damage. Based on the widely adopted definitions mentioned above, the incidence of sIUGR ranges from 10-15%. It is now generally accepted by scholars that uneven share of monochorionic twin placentas underlies the pathogenesis of sIUGR. As gestational weeks progress, ultrasound monitoring of small fetal umbilical blood flow often shows three conditions, namely normal diastolic flow, intermittent disappearance or reversal, and persistent disappearance or reversal. The clinical prognosis of sIUGR type I is better, with a stillbirth incidence of 2%-4%. Progression to type II or III is rare, and once type I is diagnosed, the majority of cases remain unchanged in fractionation before delivery. Although small fetuses develop slowly during pregnancy, the prognosis is good for both fetuses, and the incidence of neonatal neurological damage is 0%-4.3%. The clinical prognosis for sIUGR type II is poor, and 90% of patients are prone to sudden deterioration or even intrauterine death in anticipation of treatment, and need to be monitored closely after entering the perinatal period. The general termination of pregnancy is around 30 weeks, and in a few cases, termination can be maintained until 32 weeks. The incidence of neonatal neurological injury is about 14.4%. sIUGR type III has an intermediate clinical prognosis between type I and type II, but sudden and unpredictable intrauterine death occurs in about 15% of cases, some of which occur days or even hours after normal labor and delivery, and the incidence of neonatal brain injury is about 19%. Anastomotic vessels and selective intrauterine growth restriction Anastomosis is a “double-edged sword” for sIUGR. As discussed earlier, the monochorionic twin placental share and superficial anastomotic vessels affect the final distribution of blood flow to the twin fetuses. First, the anastomotic vessels have a compensatory and protective effect on sIUGR, mainly because the larger fetus can compensate the perfusion deficit of the smaller fetus due to the smaller placental share through the anastomotic vessels, and the study suggests that the difference between fetal weight is smaller than the difference between placental shares, and the two do not show a corresponding linear relationship. In addition, the study suggests that the mean duration of intrauterine survival of small fetuses with sIUGR is prolonged compared to singleton growth-restricted, fetuses or growth-restricted fetuses in twin chorionic twins, also reflecting to some extent the protective effect of anastomotic vessels on small fetuses with sIUGR. When the ratio of weight difference to placental area difference in uncomplicated monochorionic twin fetuses was 1, the ratio of weight difference to placental area difference in two fetuses with sIUGR was significantly less than 1, further reflecting the compensatory effect of anastomotic vessels on sIUGR. Second, anastomotic vessels are a potential threat to sIUGR precisely because of the presence of placental anastomotic vessels, especially the coarse AA anastomotic vessels, which can give acute intrauterine transfusion to a small fetus once the hemodynamics of the small fetus are altered, which may eventually lead to neurological injury in the large fetus. Anastomotic vascular characteristics may also differ between the 3 types of sIUGR. the characteristics of the anastomotic vessels of types I and II are similar to those of uncomplicated monochorionic twin fetuses, in that blood with a higher oxygen content can be delivered from the large fetus to the small fetus through partially bi-directional anastomotic vessels, compensating to some extent for the lack of blood volume in the small fetus due to insufficient placental share. type III differs from both types I and II in the presence of In most cases, the AA anastomotic vessels can compensate for the growth of the small fetus to a greater extent, but on the other hand, it is also due to the presence of thick AA anastomotic vessels that subtle hemodynamic changes in the small fetus may cause corresponding blood flow changes in the large fetus, resulting in serious complications such as neurological damage to the large fetus or sudden intrauterine death. Placental share, cord attachment position and selective intrauterine growth restriction As mentioned earlier, the basic anatomical basis for the occurrence of sIUGR is the uneven division of the placental share between the twins.Fick et al. suggested a 9.8-fold increased risk of inconsistent twin birth weight in cases of monochorionic twin placental share compared to cases of consistent placental share.Lewi et al. found that the difference in birth weight between the two fetuses increased as the difference in placental share increased. DePaepe et al. suggested uneven placental share and non-central attachment of the umbilical cord as risk factors for twin birth weight disparity, and Lopriore et al. found a significantly higher rate of uneven placental share in the twin birth weight disparity group compared to the uncomplicated group. Kent et al. found a significantly higher incidence of non-central attachment of the umbilical cord in small fetuses with sIUGR, with a significantly increased risk of both inconsistent birth weight and sIUGR in patients with umbilical cord sail attachment. However, it is unclear whether the uneven segmentation of the placenta during growth complicates cord non-central attachment or sail attachment, or whether cord non-central attachment or sail attachment plays an independent role in the development of sIUGR. It has been suggested that umbilical cord noncentral attachment or sail attachment may be susceptible to stress during fetal position changes and eventually cause poor fetal tissue perfusion, but large sample studies and animal models are lacking to confirm this. Treatment of selective intrauterine growth restriction and placental characteristics Overall, patients with sIUGR type I have better pregnancy outcome and generally do not require intrauterine intervention and can expect treatment under close monitoring with repeat ultrasound every 2 weeks to monitor changes in umbilical blood flow, and if no absence or inversion of umbilical blood flow occurs, pregnancy can be expected until about 34-35 weeks. For patients with sIUGR type II and III, in addition to closely monitored ultrasound for expectant treatment and timely termination of pregnancy after dexamethasone to promote fetal lung maturation, intrauterine intervention has been actively attempted by scholars in China and abroad. The more widely accepted indications are a growth-restricted fetus with absent or inverted venous catheter a-wave, a high risk of intrauterine fetal death, and, within ethical tolerances, the willingness of the patient and family to operate. There are two main methods: 1. Selective fetal reduction: radiofrequency ablation, bipolar electrocoagulation or umbilical cord ligation under fetoscopy can be applied under ultrasound guidance to block the blood flow of the umbilical cord to reduce the small fetus and avoid the neurological damage that may be caused by acute blood loss in the large fetus due to sudden intrauterine death of the small fetus, and to protect the large fetus and prolong its terminating gestational week to some extent. The procedure is relatively simple, and the postoperative live birth rate of surviving fetuses is reported to be 80%-85% in the literature. 2. Fetoscopic laser electrocoagulation of placental anastomotic vessels: it can block the superficial placental anastomotic vessels and try to preserve both fetuses.