The inter-iliac crest diameter is measured as follows: the pregnant woman is positioned in the supine position with her legs extended, and the widest distance between the outer edges of the two iliac crests is measured by the pelvimetry instrument, with a normal value of 25-28 cm. If the value is severely small or large, it is necessary to consult a doctor in time to clarify the specific situation. The iliac crest is the upper edge of the wing of the ilium bone, the protrusion at its anterior end is called the anterior superior iliac spine, and the protrusion at its posterior end is called the posterior superior iliac spine, and the highest point of the right and left iliac crests connects to the line leveling the spinous processes of the 4th lumbar vertebrae. Or between the third and fourth lumbar vertebrae.