1 , Data and methods 1.1 General data Thirty-six patients diagnosed with TOF by fetal MRI and postpartum echocardiography were studied. Gestational age at the time of fetal echocardiography ranged from 20 to 36 weeks, with an average of (27.3±4.2) weeks, and maternal age ranged from 21 to 36 years, with an average of (26.3±4.5) years. 1.2 Study methods 1.2.1 Prenatal ultrasound diagnosis A PHILIPS SONOS 5500, iE33 color Doppler echocardiograph, S4, S8, S8-3, S5-1, C5-1 probes with probe frequencies from 1.0 to 8.0 MHz was used. pregnant women were placed in the supine position with the abdomen exposed and the probe was swept systematically over the surface of the pregnant women’s abdomen. The fetal orientation, heart position, atrial position and ventricular position were determined according to the overall fetal examination procedure and the fetal heart sequence segmental diagnosis method, and the connection between atria, ventricles and aorta was analyzed, and the presence of atrial and ventricular septal defects and abnormalities of cavernous and pulmonary venous return were observed to finally establish the diagnosis [8]. The following structures are highlighted: ①Four-chamber heart section: observe the proportions of the heart, chest and each heart chamber, the position of the heart, the orientation of the apex and the angle of the heart axis; whether the foramen ovale is open and the orientation of the foramen ovale valve; whether there is interruption of the septal echo; and the morphology and function of the atrioventricular valve. ②Right and left ventricular outflow tract views: observe whether the two are crossed; measure the internal diameter of the aorta and pulmonary artery, and color Doppler can observe the flow signal and spectral characteristics of the aortic and pulmonary valve orifices. ③Three-vessel view: determine the size and arrangement relationship of pulmonary artery, aorta and superior vena cava; color Doppler can observe whether there is abnormal blood flow in the arterial ductal arch and aortic arch. 1.2.2 Fetal MRI examination 16 patients underwent fetal MRI examination within 24 h after prenatal ultrasound diagnosis. Pregnant women were placed supine on the MRI examination bed, and two-dimensional fast equilibrium steady-state feed sequences and single excitation fast self-selected echo sequence sweeps [9-10] were performed using a GE Echospeed 1.5T MRI (8-channel cardiac coil) and a Philips Achieva 1.5T MRI (16-channel SENSE-XL-Torso coil), and the diagnosis was established. 1.2.3 Postnatal echocardiography All 36 fetuses underwent routine transthoracic echocardiography after delivery in our hospital, and the diagnosis was established according to the sequential segmental diagnostic method of cardiac ultrasound [11]. 2. Results 2.1 Echocardiographic signs The main echocardiographic signs were aortic span, poorly aligned ventricular septal defect, and pulmonary stenosis. In 36 patients, the right and left hearts were basically symmetrical, and the cardiothoracic ratio was basically normal (Figure 1,2). 6 patients showed slight hypertrophy of the right ventricle; 35 patients showed intact ventricular septum in this view, and only 1 patient with combined complete atrioventricular septal defect showed atrioventricular access type ventricular defect; 13 patients showed tricuspid regurgitation. (ii) Left ventricular outflow tract views: 35 patients showed malaligned ventricular septal defects; the aorta was widened and rode over the ventricular septum, showing a special “flare sign” (Figure 3); one fetus failed to show the left ventricular outflow tract view. (iii) Right ventricular outflow tract views: 29 cases showed pulmonary valve stenosis with hypoplastic trunk and branches; 4 patients with combined pulmonary valve agenesis showed small pulmonary valve annulus and dilated common pulmonary trunk and branches; Doppler echocardiography revealed increased pulmonary artery flow velocity in only 5 patients, and 4 patients with combined pulmonary valve agenesis had massive regurgitation (Figures 4 and 5); 2 cases were diagnosed with normal pulmonary artery development. One fetus failed to show a right ventricular outflow tract view. Three-vessel views showed normal arrangement of the pulmonary artery, aorta, and superior vena cava. 31 patients without combined pulmonary valve defect showed pulmonary artery stenosis with a typical “?” sign (Figure 6); one fetus failed to show a three-vessel view. 2.2 Ultrasound diagnosis In comparison with the results of fetal MRI and postnatal ultrasound (Figure 7, 8), the prenatal ultrasound diagnosis was correct in 31 of 36 patients (86.1%); misdiagnosis was made in 4 cases (11.1%), including 2 cases of ventricular septal defect and 2 cases of right ventricular double outlet. The diagnosis was made at 36 weeks of gestation when the echocardiogram was repeated. Postnatal echocardiography revealed three cases with multiple myocardial septal defects and one case with coronary artery malformation (the left anterior descending branch of the left coronary artery originated from the right coronary artery), which were not diagnosed by prenatal ultrasound. 2.3 Combined anomalies The most common combined anomalies in TOF fetuses were tricuspid regurgitation in 13 cases (36.1%), aortic regurgitation in 6 cases (16.7%), pulmonary valve defect in 4 cases (11.1%), right aortic arch in 4 cases (11.1%), left superior vena cava remnant in 3 cases (8.3%), and atrial septal defect in 1 case (2.8%). The main pathological changes of TOF were poorly aligned ventricular septal defect, aortic span, pulmonary artery stenosis and right ventricular hypertrophy. Since the blood flow in the body circulation during fetal life mainly comes from the right heart system, the intraventricular right-to-left shunt caused by pulmonary artery stenosis, ventricular septal defect and aortic span does not affect the overall fetal development; right ventricular hypertrophy is not obvious in early and middle pregnancy, and the four-chamber cardiac section often shows symmetry of both ventricles and intact septum at this time, so it is easy to be missed. In cases of severe pulmonary stenosis during pregnancy, secondary right ventricular hypertrophy may occur due to heavy right ventricular afterload. In this group, only 6 cases showed slight hypertrophy of the right ventricle, and 1 case was misdiagnosed as a normal fetus in early pregnancy because the four-chamber cardiac section was normal and exhaustive echocardiography was not performed; at 36 weeks of pregnancy, the right ventricle was obviously hypertrophied and exhaustive prenatal ultrasonography was performed, and the diagnosis of TOF was made by finding septal defect, aortic span and pulmonary artery stenosis. Although it can show most of the structures of the heart, it cannot show the outflow tract and has less value in the diagnosis of fetuses with conus arteriosus stem anomalies such as TOF [1,12]. The International Society of Obstetrics and Gynecology specifies that the routine screening cardiac views in mid-pregnancy are the four-chamber cardiac view, outflow tract view and three-vessel view. malformations; the detection rate of congenital heart disease by the combined application of the three views is more than 85% [2]. In our group, 35 patients showed malaligned ventricular septal defect and aortic span in the left ventricular outflow tract view and were able to determine the degree of aortic span; 33 patients showed pulmonary artery stenosis in the right ventricular outflow tract view, and the degree of pulmonary artery stenosis and regurgitation could be further determined by combining color and spectral Doppler ultrasound. This indicates that the left and right ventricular outflow tract views are the most critical views for the diagnosis of TOF. Pulmonary artery stenosis is the most important pathological manifestation of TOF. In this group of 2 patients with TOF, the presence of pulmonary stenosis could not be clearly identified due to the small fetal pulmonary valve device, which did not show the opening and closing activity of the pulmonary valve during echocardiography, and only a ventricular septal defect was diagnosed. However, when we looked back at the video, we found that in the three-vessel view, although the pulmonary artery, aorta, and superior vena cava were arranged in a normal order, the pulmonary artery was smaller than the internal diameter of the aorta, and the normal V sign disappeared and showed a “? sign [13]. It is suggested that triple-vessel views have the advantage of visualization in determining whether the pulmonary artery is stenosed, and when the pulmonary artery may be smaller than the aorta with the naked eye, the internal diameter of both should be carefully measured. The three-vessel view can also determine whether there is a residual left superior vena cava and, in combination with the position of the trachea, whether it is a left or right aortic arch, etc. It is an important complement to the four-chamber cardiac view and the ventricular outflow tract view. Because of the presence of ventricular septal defect and aortic riding in both right ventricular double outlet and TOF, it is easy to misdiagnose. It is generally accepted that if there is a fibrous connection between the anterior aortic wall and the anterior mitral valve, it is more likely to be a TOF, and if there is a muscular connection between the two, it is more likely to be a right ventricular double outlet. Because of the small fetal heart structure during pregnancy and the variable body position, it is difficult to identify the fetus during the fetal period. In this group of 2 TOF fetuses, the right ventricular double outlet was misdiagnosed prenatally. The right heart system is predominant during fetal life, and physiologic regurgitation is often present due to high pulmonary artery pressure and is characterized by short, thin and narrow regurgitant bundles with short duration [5]. Thirteen of our patients combined with different degrees of tricuspid regurgitation, but there were no significant changes in the valve device and no significant aggravation after birth, which was considered to be due to right ventricular outflow tract obstruction causing increased right ventricular pressure. However, a recent study has shown that the presence of tricuspid regurgitation in early pregnancy is associated with the development of precordial disease, and the combination of posterior nuchal translucency thickening and the appearance of a reverse A-peak in venous catheter flow may improve the detection rate of precordial disease [14]. It is suggested that tricuspid regurgitation during pregnancy should be taken seriously and a detailed evaluation of the tricuspid valve device should be performed to determine the nature of tricuspid regurgitation and whether it is combined with other cardiovascular malformations. Pulmonary valve agenesis is a rare congenital cardiovascular malformation, often combined with TOF, accounting for approximately 2.4% to 6.3% of cases. The embryologic developmental mechanism is unknown and was previously thought to be due to arteriovenous ductal agenesis during embryonic development. In contrast, a recent study found that some fetuses with pulmonary valve agenesis in early pregnancy keep their ductus arteriosus open and often die of severe cardiac insufficiency; in mid- to late-pregnancy, the ductus arteriosus closes and is clinically mistaken for ductus arteriosus [15]. In our group of 36 patients with TOF, 4 cases combined with pulmonary valve agenesis, accounting for 11.1%; none of the ultrasound examinations revealed an open arterial duct, which may be related to the fact that all patients in this group were in mid- to late-pregnancy and the open arterial ducts were closed. It is suggested that arteriovenous ductus arteriosus may not be a necessary condition for pulmonary valve agenesis, but only a clinical sign of pulmonary valve agenesis. Patients with TOF often have combined myocardial ventricular septal defects and coronary artery malformations, which are more frequently reported on postnatal ultrasonography and cardiovascular angiography. Due to the predominance of the right ventricle and high pulmonary artery pressure, small shunts in both directions at the ventricular level are often not visualized and the myocardial ventricular defect is easily missed. Because of the small coronary arteries and the inability of ultrasound to show the entire coronary artery pathway, coronary artery malformations are easily missed by ultrasound examination. In our group of 36 fetuses, 3 cases were combined with myoventricular septal defect and 1 case was combined with coronary artery malformation, all of which were missed, indicating that although the resolution of ultrasound probe has been improved, fetal ultrasound examination still has certain limitations, and the examiner should be vigilant. Based on the above results, this study concluded that the combined application of four-chamber cardiac section, ventricular outflow tract section and three-vessel section, based on the signs of malaligned ventricular septal defect, aortic span and pulmonary artery stenosis, echocardiography can diagnose fetal TOF more accurately, but it is easy to miss the diagnosis of myocardial ventricular septal defect and coronary artery malformation, and it is difficult to differentiate from right ventricular double outlet. The shortcomings of this study are: (i) the number of cases is small; (ii) the results were only compared with fetal MRI and postnatal ultrasonography, but not with surgical or pathological anatomical diagnosis, which may have some bias. In the future, we will expand the sample size and conduct further in-depth studies.