<prompt> Smallest pelvic diameter in anteroposterior plane of inlet: March 2009Obstetric conjugateDiagonal conjugateTrue conjugatetransverse conjugate  <response>: Ans. A: Obstetric Conjugate Antero -posterior diameters: Anatomical antero-posterior diameter true conjugate 11cm-from the tip of the sacral promontory to the upper border of the symphysis pubis. Obstetric conjugate 10 cm-from the tip of the sacral promontory to the most bulging point on the back of symphysis pubis which is about 1 cm below its upper border. It is the shoest antero-posterior diameter. Diagonal conjugate 12 cm-1.5 cm longer than the true conjugate. From the tip of sacral promontory to the lower border of symphysis pubis. External conjugate 20 cm-from the depression below the last lumbar spine to the upper anterior margin of the symphysis pubis measured from outside by the pelvimeter. It has not a true obstetric impoance. Transverse diameters: Anatomical transverse diameter 13cm -between the fahest two points on the iliopectineal lines. It lies 4 cm anterior to the promontory and 7 cm behind the symphysis. It is the largest diameter in the pelvis. Obstetric transverse diameter: It bisects the true conjugate and is slightly shoer than the anatomical transverse diameter. Oblique diameters: Right oblique diameter 12 cm -from the right sacroiliac joint to the left iliopectineal eminence. - Left oblique diameter 12 cm -from the left sacroiliac joint to the right iliopectineal eminence. Sacro-cotyloid diameters 9-9.5 cm -from the promontory of the sacrum to the right and left iliopectineal eminence, so the right diameter ends at the right eminence and vice versa. 