Obstetrical conjugate is estimated indirectly from

Four diameters of the pelvic inlet are usually described: anteroposterior, transverse, and two oblique diameters. Most cephalad, the anteroposterior diameter, termed the true conjugate, extends from the uppermost margin of the symphysis pubis to the sacral promontory. The clinically impoant obstetrical conjugate is the shoest distance between the sacral promontory and the symphysis pubis. Normally, this measures 10 cm or more, but it cannot be measured directly with examining fingers. Thus, for clinical purposes, the obstetrical conjugate is estimated indirectly by subtracting 1.5 to 2 cm from the diagonal conjugate, which is determined by measuring the distance from the lowest margin of the symphysis to the sacral promontory (Ref: William's Obstetrics; 25th edition)