Excerpt from Obstetrical Emergencies: By an Experienced Obstetrician The true conjugate can be estimated from the diag onal conjugate (see Pelvic Planes and Fig. By sub tracting from it 1 inch but the amount will vary with the height, thickness and inclination of the symphysis, and the height of the sacral promontory. Measurements deducted vary from to 1 inches. Williams states from to inch