<prompt> Shortest anteroposterior diameter of pelvic inlet isObstetric conjugateDiagonal conjugateAnatomic conjugateAll are equal  <response>: Pelvic InletAlso called the superior straitPelvic inlet is also the superior plane of the true pelvisBounded posteriorly by the promontory and alae of the sacrum, laterally by the linea terminalis, and anteriorly by the horizontal pubic rami and the symphysis pubisDuring labour, fetal head engagement is defined by the fetal heads biparietal diameter passing through this planeFour diameters of the pelvic inlet are usually described: anteroposterior, transverse, and two oblique diametersDistinct anteroposterior diameters have been described using specific landmarksMost cephalad, the anteroposterior diameter, termed the true conjugate, extends from the uppermost margin of the symphysis pubis to the sacral promontoryThe 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 