Obstetrical pelvic measurements

The figures for the measurements of the inlet, mid-cavity and outlet of the true pelvis are readily committed to memory in the form shown in Table 3.

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. This normally measures 5in (12.5 cm); from the

Table 2 Comparison of male and female pelvis.

Male

Female

General structure Joint surfaces Muscle attachments False pelvis Pelvic inlet Pelvic canal

Pelvic outlet 1st piece of sacrum

Sacrum

Sacroiliac articular facet

(auricular surface) Subpubic angle (between inferior pubic rami) Inferior pubic ramus

Acetabulum Ischial tuberosities Obturator foramen

Heavy and thick Large

Well marked Deep

Heart shaped

'Long segment of a short cone' i.e. long and tapered Comparatively small The superior surface of the body occupies nearly half the width of sacrum

Long, narrow, with smooth concavity

Extends well down the 3rd piece of the sacrum

'The angle between the middle and index finger' Presents a strong everted surface for attachment of the crus of the penis Large Inturned Round

Light and thin Small

Rather indistinct

Shallow

Oval

'Short segment of a long cone' i.e. short with almost parallel sides Comparatively large Oval superior surface of the body occupies about one-third the width of sacrum Short, wide, flat, curving forward in lower part Extends down only to upper border of

3rd piece 'The angle between the thumb and index finger' This marking is not present

Small

Everted

Oval

Fig. 95 The measurements of the female pelvis. (a) The inlet, (b) the outlet. (c) Lateral view to show the diagonal conjugate.

Fig. 95 The measurements of the female pelvis. (a) The inlet, (b) the outlet. (c) Lateral view to show the diagonal conjugate.

Diagonal Conjugate
Table 3 Obstetrical pelvic measurements.

Transverse

Oblique

Anteroposterior

Inlet

5 in (12.5 cm)

4.5 in (11.5 cm)

4 in (10 cm)

Mid-pelvis

4.5 in (11.5 cm)

4.5 in (11.5 cm)

4.5 in (11.5 cm)

Outlet

4 in (10cm)

4.5 in (11.5 cm)

5 in (12.5 cm)

practical point of view, it is not possible in the normal pelvis to reach the sacral promontory on vaginal examination either readily or without discomfort to the patient.

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.

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