This question examines the pelvic diameter line and prompts you to remember the relevant values. The pelvic entrance plane is transverse oval with four main diameter lines, among which the anterior and posterior diameter of the entrance plane is also called true combined diameter, and the average length is 1 1cm. The plane of the middle pelvis is the narrowest plane of the pelvis, which is oval vertically and has two main diameter lines. The average length of the middle pelvis is11.5 cm. The transverse diameter of the middle pelvis, also known as the diameter between ischial spines, is 10cm on average.