Conjugate of the diagonal is taken as 'a' cm, then obstetric conjugate will be
Obstetrical conjugate

It 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
a + 1 cm
a + 2 cm
a - 1 cm
a - 2 cm