The obstetric conjugate diameter is the distance between the sacral promontory and the:

Obstetric conjugate is the distance between the sacral promontory and the .nearest point on the posterior surface of the symphysis pubis.   Important points:       Obstetric conjugate measures 10 cm, It cannot be clinically measured but is to be inferred from diagonal conjugate or by lateral radiopelvimetry.       True/anatomical conjugate is the distance between midpoint of sacral promontory to the inner margin of CT. upper border of symphysis pubis. It measures 11 cm, Its measurement is inferred by subtracting 1.2 cm from ft diagonal conjugate.        Diagonal conjugate is the distance between the lower border of symphysis pubis to the midpoint on the s promontory. It is measures 12 cm. It is measured clinically during pelvic assessment.        Obstetric conjugate can be calculated by subtracting 1.5-cm from the diagonal conjugate.