Shortest anteroposterior diameter of pelvic inlet is
Pelvic Inlet

Also called the superior strait
Pelvic inlet is also the superior plane of the true pelvis
Bounded posteriorly by the promontory and alae of the sacrum, laterally by the linea terminalis, and anteriorly by the horizontal pubic rami and the symphysis pubis
During labour, fetal head engagement is defined by the fetal head’s biparietal diameter passing through this plane
Four diameters of the pelvic inlet are usually described: anteroposterior, transverse, and two oblique diameters
Distinct anteroposterior diameters have been described using specific landmarks


Most cephalad, the anteroposterior diameter, termed the true conjugate, extends from the uppermost margin of the symphysis pubis to the sacral promontory
The clinically important obstetrical conjugate is the shortest 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

       7. The transverse diameter is constructed at right angles to the obstetrical conjugate and represents the greatest distance between the linea terminalis on either side. It usually intersects the obstetrical conjugate at a point                                 approximately 5 cm in front of the promontory and measures approximately 13 cm
       8. Each of the two oblique diameters extends from one sacroiliac synchondrosis to the contralateral iliopubic eminence. Each eminence is a minor elevation that marks the union site of the ilium and pubis. These oblique diameters                   average less than 13 cm
Obstetric conjugate
Diagonal conjugate
Anatomic conjugate
All are equal