In obstetrics the inlet to the true pelvis, bounded by the sacral promontory, the horizontal rami of the pubic bones, and the top of the symphysis pubis. Because the infant must pass through the inlet to enter the true pelvis and to be born vaginally, the anteroposterior, transverse, and oblique dimensions of the inlet are important measurements to be made in assessing the pelvis in pregnancy.
There are three anteroposterior diameters: the true conjugate, the obstetric conjugate, and the diagonal conjugate. The true conjugate can be measured only on radiographic films because it extends from the sacral promontory to the top of the symphysis pubis. Its normal measurement is 11 cm or more. The obstetric conjugate is the shortest of the three. It extends from the sacral promontory to the thickest part of the pubic bone and measures 10 cm or more. The diagonal conjugate is the most easily and commonly assessed because it extends from the lower border of the symphysis pubis to the sacral promontory. It normally measures 11.5 cm or more. The inlet is said to be contracted when any of these diameters is smaller than normal.
METHOD OF VAGINAL EXAMINATION TO DETERMINE THE DIAGONAL CONJUGATE:
In order to reach the sacral promontory, the examiner's elbow must be flexed and the perineum forcibly indented by the knuckles of the third and fourth fingers. The index and the second fingers are carried up and over the anterior surface of the sacrum. By deeply inserting the wrist, the promontory may be felt by the tip of the second finger as a projecting bony margin. With the finger closely applied to the most prominent portion of the upper sacrum, the vaginal hand is elevated until it contacts the pubic arch. The immediately adjacent point on the index finger is marked, as shown in First figure
. The distance between the mark and the tip of the second finger is the diagonal conjugate. The obstetrical conjugate is computed by subtracting 1.5 to 2.0 cm, depending on the height and inclination of the symphysis pubis.Normally the obstetrical conjugate measures 10 cm or more.
If the diagonal conjugate is greater than 11.5 cm, it is justifiable to assume that the pelvic inlet is of adequate size for vaginal delivery of a normal-sized fetus.
The anteroposterior diameter of the pelvic inlet that has been identified as the true conjugate does not represent the shortest distance between the promontory of the sacrum and the symphysis pubis .The shortest distance is the obstetrical conjugate, which is the shortest anteroposterior diameter through which the head must pass in descending through the pelvic inlet.