Objective To explore the flow chart and key points of etiological differential diagnosis of character"卜"sign in fetal three-vessel and trachea(3VT)view.Methods A retrospective study was performed on ultrasound images of 37 fetuses,who had"卜"sign in 3VT view during fetal heart ultrasound scanning in The Affiliated Suzhou Hospital of Nanjing Medical University from Aug.2020 to Oct.2023.The types of diseases with"卜"sign and the flow chart and key points of differential diagnosis were summarized.All cases were diagnosed by 2 experienced senior prenatal ultrasound doctors independently.Results The incidence of"卜"sign was 0.14％(37/27 019).Among the 37 fetuses with"卜"sign on 3VT view,13(35.14％)cases had complete transposition of great arteries(TGA),11(29.73％)had double outlet of right ventricle with abnormal relationship of great arteries(DORV/AA),7(18.92％)had truncus arteriosus(TA,van Praagh A1,A2 and A3),and 6(16.22％)had pulmonary atresia with ventricular septal defect(PA/VSD).During the process of differential diagnosis,the presence or absence of aorta and pulmonary arteries,the ventricle-artery connection relationship,and the blood supply to both lungs should be focused on,and the final diagnosis can be derived step by step according to the flow chart.The key points of differential diagnosis were as follows:the ventricle-artery connection was inconsistent in TGA;in DORV/AA both aorta and pulmonary artery were entirely or mostly from right ventricle without normal spiral relationship;in TA only a common artery trunk was detected,and main pulmonary artery or its branches were from the artery trunk;and in PANSD pulmonary artery was atresia and only aorta was detected,and the blood supply to both lungs was from ductus arteriosus and/or aortopulmonary collateral arteries.Conclusion The fetal differential diseases with"卜"sign in 3VT view include TGA,DORV/AA,TA(van Praagh A1,A2 and A3),and PA/VSD.Understanding the flow chart and key points of differential diagnosis can improve the prenatal detection rate and diagnosis of related malformations.