(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. The anteroposterior diameters are shorter than normal in the small gynecoid and platypelloid pelvis. The transverse diameter of the inlet is bounded by the inferior border of the walls of the iliac bones and is measured at the widest point. It is normally close to 13.5 cm but may be less in the small gynecoid pelvis and anthropoid pelvis. The oblique diameters of the pelvis extend from the juncture of the sacrum and ilium to the eminence on the ilium on the opposite side of the pelvis. Each oblique diameter measures nearly 13 cm. This dimension is smaller than normal in the small gynecoid and platypelloid pelves.

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