The smallest diameter of the true pelvis is
The interspinous diameter is 10 cm or slightly greater, is usually the smallest pelvic diameter and in cases of obstructed labor, is paicularly impoant Inteuberous (11 cm or 4 1/4 ''): It measures between inner borders of ischial tuberosities. Diagonal conjugate: It is the distance between the lower border of symphysis pubis to the midpoint on the sacral promontory. It measures 12 cm (4 3/4 '') Anteroposterior (Syn : true conjugate, anatomical conjugate, conjugate vera) : It is the distance between the midpoint of the sacral promontory to the inner margin of the upper border of symphysis pubis. It measures 11 cm (4 1/4 ''). It is not the shoest diameter of the inlet in the anteroposterior plane. In practice, the true conjugate cannot be estimated directly. However, its measurement is inferred by subtracting 1.2 cm ( 1/2 '') from the diagonal conjugate thus allowing for the inclination, thickness and height of the symphysis pubis. Reference: William's Obstetrics; 24th edition; Chapter 1
Interspinous diameter
Diagonal conjugate
True conjugate
Inteuberous diameter
General obstetrics