We found that the (CRL-URL)/CRL and URL/CRL ratios significantly differed between the survival and non-survival groups at 11–14 weeks of gestation. These results were consistent with the findings from Roethlisberger et al. in which the (CRL-URL)/CRL and URL/CRL ratios were potential predictors of TRAP pregnancy outcomes during early gestational ages .
Several studies have shown that the size of the acardiac twin was associated with the outcomes of the pump twin [2, 4, 5]. Moore et al. summarized the clinical outcomes of 49 cases of TRAP . When the ratio of the acardiac twin body weight to the pump twin body weight was more than 70%, the risk of an adverse outcome for the pump twin increased. Jelin et al. analyzed 18 acardiac twins whose body weights were no more than 50% of those of the pump twins; they found that only one pump tiwn with MCMA twins died in utero . One study conducted in the Children's Hospital of Philadelphia, investigating 13 cases of TRAP, found that when the body weights of the acardiac twins were no more than 40% of those of the pump twins, no edema and heart failure were seen in the pump twins . These findings were consistent with our findings. In the first trimester, it is easy to conduct the measurement of the fetal length using an ultrasound. Additionally, it can provide potential predictors to guide the clinical management of TRAP during early gestational ages.
The treatment of TRAP aims to increase the pump twin’s survival rate. The key treatment method is to block the blood flow of the acardiac twin and allow the pump twin to recover from the abnormal circulation. Snijders et al. found that TRAP can be diagnosed at 11–14 weeks of gestation, supporting the need for the early clinical management of TRAP . However, there is no consistent management for TRAP. Some specialists prefer to perform expectant management [4, 5]. Several studies have indicated that the intrauterine mortality rate of the pump twins after expectant management ranged from 43 to 100% [1, 3]. The timely use of micro-invasive therapy in utero such as radiofrequency ablation and bipolar cord coagulation can increase the survival rate of the pump twin to some extent. The International Society of Ultrasound in Obstetrics and Gynecology demonstrated that it was feasible to perform micro-invasive preventive surgery on cases with TRAP before 16 weeks of gestation . However, some specialists are concerned that intrauterine therapy for the fetus with a gestational age of 16 weeks may cause the PROM, maternal pulmonary edema, placental damage, and that it may also increase the risk of skin damage and neural damage of the pump twin, as well as the death rate of the pump twin [1, 8, 9]. Therefore, it is pivotal to establish reasonable management strategies for TRAP.
Various studies suggested that expectant management may be a good management strategy for TRAP. One study conducted in Australia found that the survival rate of the pump twin was more than 80% after the expectant management of TRAP . In our study, 21 cases received expectant management and the survival rate of the pump twin was 66.7%.
Some studies suggested that an invasive intrauterine intervention may be conducted in the pump twin if discovered that there is polyhydramnion, the abdominal circumference of the acardiac twin is equal to or greater than that of the pump twin, or the pump twin has severe abnormal blood flow [4, 11, 12]. Severe abnormal blood flow was defined as blood flow in the umbilical artery reversed or disappeared at the diastolic phase, pulsatory blood flow of the umbilical vein or DV in the reverse direction, a peak systolic velocity of the middle cerebral artery > 1.5 MOM, or a pulse index of DV > 95%. In our study, when edema, heart failure, and polyhydramnion in the pump twin occurred or the body weight of the acardiac twin was greater than 50% of that of the pump twin, it was recommended that invasive management be performed. Among the 21 cases, six cases were subjected to bipolar electrocoagulation in the umbilical cords of the acardiac twins at 24–26 weeks of gestation. Overall, 66.7% (n = 14) of the pump twins experienced a live birth.
In some TRAP cases, the blood flow of the acardiac twins may be naturally blocked in development. Chen et al. reported that there were seven cases of naturally blocked TRAP, with three cases of intrauterine death and one case of polyhydramnion-induced premature birth . In our study, we identified five TRAP cases with naturally blocked blood flow, of which four cases survived. In one case, the pump twin had polyhydramnion within 1 week after the natural blocking of the blood flow. The pressure in the umbilical blood vessels abruptly decreased after the natural blocking of the blood flow of the acardiac twin. The blood flow of the pump twin was perfused to the acardiac twin, thereby increasing the cardiac burden on the pump twin. The pump twin consequently developed heart failure with a high cardiac output, and thereafter died of heart failure. Therefore, ultrasounds can be used in TRAP cases to monitor the development of the pump twin after the natural blood flow has been blocked.
Some studies have indicated that the resistance index of the umbilical artery and the pulsatility index of the middle cerebral artery may be indicators to assess the prognosis of TRAP [14, 15]. However, these indicators for blood flow in TRAP were detected at gestational ages of greater than 20 weeks; thus, some important indicators may have eluded observation in the first trimester. Therefore, high-quality studies are required in the future to explore the ability of ultrasounds to predict adverse outcomes of TRAP in the first trimester.