Critical obstetric conjugate for trial of labour is :

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 It is the most impoant AP diameter of the pelvic inlet as it is the one through which the fetus must pass It is the smallest AP diameter The pelvic inlet is considered to be contracted if obstetric conjugate is less than 10cm Reference: William&;s Obstetrics; 24th Edition; Chapter 1