Main findings
In this study, the anatomical structures of the vagina were distinctly displayed on MRI. A reference line that passes through the inferior aspect of the pubic symphysis to the midpoint of the third sacral vertebra was introduced and named PS3L. The marker points representing the distal, middle, and apical points of the upper two-thirds of the vagina showed relatively concentrated distributions along this line, and the median distances from these points to the line were -0.5 cm, 0.0 cm, and -0.2 cm. Additionally, the vaginal axis lay nearly parallel to this line, and the median angle between the line and the vaginal axis was 0.0 degree. These trends were observed in all age subgroups, indicating that the reference line PS3L approximates the axis of the normal upper two-thirds of the vagina.