Pelvic Diameters:
Knowledge of the diameters of the normal female pelvis is essential in the practice of midwifery because contractions of any of them can result in malposition or malpresentation of the presenting part of the fetus.
Diameters of the pelvic inlet:
The brim has four principal diameters: the anteroposterior diameter, the transverse diameter and the two oblique diameters.
The anteroposterior or conjugate diameter extends from the mid point of the sacral promontory to the upper border of the symphysis pubis. Three conjugate diameters can be measured: the anatomical conjugate, the obstetrical conjugate and the internal and diagonal conjugate.
The anatomical conjugate, which average 12 cm, is measured from the sacral promontory to the uppermost point of the symphysis pubis.
The obstetrical conjugate, which average 11 cm, is measured from the sacral promontory to the posterior border of the upper surface of the symphysis pubis. This represents the shortest anteroposterior diameter through which the fetus must pass and is hence of clinical significance to midwives . The obstetrical conjugate can’t be measured with the examining fingers or any other technique.
The diagonal conjugate is measured anteroposterior from the lower border of the symphysis to the sacral promontory.
The transverse diameter is constructed at right angles to the obstetric conjugate and extends across the greatest width of the brim; it’s average measurement is about 13 cm.
Each oblique diameter extends from the iliopectineal eminence of one side to the sacroiliac articulation of the opposite side; it’s average measurement is about 12 cm. Each takes its name from the sacroiliac joint from which its arises, so the left oblique diameter arises from the left sacroiliac joint and the right oblique from the right sacroiliac joint.
Another dimension, the sacrocotyloid, passes from the sacral promontory to the iliopectineal eminence on each side and measure 9-9.5 cm. It’s importance is concerned with posterior positions of the occiput when the parietal eminences of the fetal head may become caught.
Diameters Of The Cavity:
The pelvic cavity is a body cavity that is bounded by the bones of the pelvis. Its oblique roof is the pelvic inlet (the superior opening of the pelvis). Its lower boundary is the pelvic floor.
The cavity is circular in shape and although it is not possible to measure it’s diameters exactly, they are all considered to be 12 cm.
Diameters Of The Outlet:
The outlet, which is diamond shaped, has three diameters: the anteroposterior diameter, the oblique diameter and the transverse diameter.
The Anteroposterior Diameter:
The anteroposterior diameter extends from the lower border of the symphysis pubis to the sacrococcygeal joint. It measures 13 cm, as the coccyx may be deflected backwards during labour, this diameter indicates the space available during birth.
The Oblique Diameter:
The oblique diameter, although there are no fixed points, is said to be between the obturator foramen and the sacrospinous ligament. The measurement is taken as being 12 cm.
The Transverse Diameter:
The transverse diameter extends between the two ischial spines and measures 10 -11 cm. It is the narrowest diameter in the pelvis. The plane of least pelvic dimensions is said to be at the level of the ischial spines.
