Smallest pelvic diameter in anteroposterior plane of inlet: March 2009

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.