If the diagonal conjugate is taken as 'a' cm, then obstetrical conjugate will be :

Obstetrical conjugate It is the shoest 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 Reference: William&;s Obstetrics; 24th edition; Chapter 1