Objective To use high frequency ultrasound for scanning the lower uterine segment during the third trimester of scar uterus pregnancy, so as to provide reference for selecting the way of delivery. Methods Totally 118 scar uterus pregnancy cases were selected. Transabdominal ultrasound scan was done for the fetus between 36^th and 40^th week of pregnancy, and then the transducer was switched to a high frequency of 7.5-12 MHz to observe the lower uterine segment. The scan results were compared with the observations of incomplete rupture of lower uterine segment and defective scars in the repeated caesarean section. Results Three types of high frequency ultrasound images were found for the lower uterine segment during the third trimester of scar uterus pregnancy: truncation sign (6.8%,8/118), rat tail sign (44.1%,52/118), and homogeneous echoes (49.1%,58/118). The diagnosis efficacies of truncation sign for incomplete rupture of uterus, rat tail sign for defective scars, and homogeneous echoes for intact caesarean scars were significantly higher than those of the other two types (χ2 valued 20.42, 19.03, and 33.08, respectively, P＜0.001). The diagnostic sensitivity, specificity, positive predictive value and negative predictive value of the truncation sign were 80.0%, 96.5%, 50.0%, and 99.0%; those of the rat tail sign were 70.0%, 73.8%, 67.3%, and 76.2%; and those of homogeneous echoes were 72.9%, 74.6%, 74.1%, and 73.3%, respectively. Conclusion High frequency ultrasound scan can display the changes of the scars on the lower uterine segment, and may serve as an effective method to diagnose incomplete rupture of uterus, defective and intact scars in the third trimester pregnancy of a caesarean-scarred uterus. To avoid uterine rupture, vaginal trial of labor should be carefully chosen for patients showing truncation sign or rat tail sign.