• The transverse diameter of the outlet is assessed clinically by measuring the distance between the ischial tuberosities along a plane passing across the anus;• The anteroposterior outlet diameter is measured from the pubis to the sacrococcygeal joint.• The most useful measurement clinically is, however, the diagonal conjugate—from the lower border of the pubic symphysis to the promontory of the sacrum.
(a) The inlet, (b) the outlet.The measurements ofthe female pelvis.
Transverse diameter AP diameterInlet 13 cm 11 cmMid pelvis 12 cm 12 cmOutlet 11 cm 13 cm
(c) Lateral view to show thediagonal conjugate.
• Another useful clinical guide is the subpubic arch: the examiner’s four knuckles (i.e. his clenched fist) should rest comfortably between the ischial tuberosities below the pubic symphysis.• Note that these measurements are all of the bony pelvis; the ‘dynamic pelvis’ of the birth-canal, in fact, is narrowed by the pelvic musculature, the rectum and the thickness of the uterine wall. Today accurate imaging techniques enable exact measurements to be made of the bony pelvis.
• 1 The normal and its variants• (a) Gynaecoid—normal.• (b) Android—the masculine type of pelvis.• (c) Platypelloid—shortened in the anteroposterior diameter, increased in the transverse diameter (the ‘non-rachitic flat pelvis’).• (d) Anthropoid—resembling that of an anthropoid ape with a much lengthened anteroposterior and a shortened transverse diameter.Variations of thepelvic shape
Pelvic variationsand abnormalities—shown as diagrammaticoutlines of the pelvicinlet.
• 2◊◊Symmetrically contracted pelvis• That of a small woman but with a symmetrical shape.• 3◊◊The Rachitic flat pelvis• The sacrum is rotated so that the sacral promontory projects forward and the coccyx tips backwards. The anteroposterior diameter of the inlet is therefore narrowed, but that of the outlet is increased. This deformity is typical of rickets, the result of vitamin D deficiency.• 4◊◊The asymmetrical• Asymmetry can be due to a variety of causes such as scoliosis, longstanding hip disease (e.g. congenital dislocation), poliomyelitis, pelvic frac ture, congenital abnormality due to thalidomide and the Naegele pelvis which is due to the congenital absence of one wing of the sacrum or its destruction by disease.