2021 Volume 81 Issue 3 Pages 242-249
The purpose of this study was to develop a method for visualizing the position of station±0 on translabial ultrasonography and to investigate its relationship with delivery outcomes. By using 501 pelvic sagittal section radiography images of Japanese pregnant women with a singleton fetus in the normal cephalic position at 35–39 weeks, the straight line connecting the projected ischial spine and inferior margin of the pubis was used as the plane of station±0, and the angle between this line and the long axis of the pubis was defined as the 0-angle. The intra- and inter-examiner errors of 0-angle were calculated, and the relationship between the 0-angle and maternal background factors （age, height, body mass index, and weeks of imaging） and the prognosis of delivery （cesarean section, frequency of instrumental delivery, and time required for delivery） was examined. The mean±standard deviation of the 0-angle was 118.9°±5.9°, with a normal distribution, and the intraclass correlation coefficients of the intra- and inter-examiner errors were 0.973 （95％ confidence interval ［CI］：0.950-0.986） and 0.967 （95％ CI：0.938-0.982）, respectively. None of the maternal background factors correlated with the 0-angle. The 0-angle in the cesarean section group was significantly smaller than that in the vaginal delivery group （119.1°±6.0° vs. 116.9°±5.1°）, especially in the cesarean section group （119.1°±6.0° vs. 115.9°±4.5°）. The 0-angle is a highly reproducible and versatile index that is independent of maternal body size. The 0-angle was significantly smaller in the group with delivery arrest, indicating that a shallow pelvic inlet surface tends to cause poor infant head descent. In addition, we visualized station±0 using 0-angle translabial ultrasonography, which can objectively evaluate the degree of infant head descent.