The midpelvis is measured at the level of the ischial spines, also called the midplane or plane of least pelvic dimensions (see Fig. 2-16). During labor, the degree of fetal head descent into the true pelvis may be described by station, and the midpelvis and ischial spines serve to mark zero station. The interspinous diameter is 10 cm or slightly greater and is usually the smallest pelvic diameter. The anteroposterior diameter through the level of the ischial spines normally measures at least 11.5 cm.
The pelvic outlet consists of two approximately triangular areas whose boundaries mirror those of the perineal triangle described earlier (p. 19). They have a common base, which is a line drawn between the two ischial tuberosities. The apex of the posterior triangle is the tip of the sacrum, and the (, .Iium
FIGURE 2-16 Axial view of a normal female pelvis. The clinically important obstetrical conjugate and transverse diameter of the pelvic inlet are illustrated. The interspinous diameter of the mid pelvis is also marked.
Anthropotd ,.,• Intermediates • .. 4
FIGURE 2-17 The four parent pelvic types of the Caldwell-Moloy classification. A line passing through the widest transverse diameter divides the inlets into posterior (P) and anterior (A) segments.
lateral boundaries are the sacrotuberous ligaments and the ischial tuberosities. 'he anrerior triangle is formed by the descending inferior rami of rhe pubic bones. These rami unite at an angle of 90 [0 100 degrees ro form a rounded arch under which the fetal head must pass. Unless there is Signiicant pelvic bony disease, rhe pelvic ourler seldom obstructs vaginal delivery.
The Caldwell-Moloy (I933, 1934) anaromical classiication of rhe pelvis is based on shape, and its concepts aid an understanding of labor mechanisms. Speciically, the grearesr transverse diameter of rhe inler and its division into alHerior and posterior segmelHs are used to classiy rhe pelvis as gynecoid, anthropoid, android, or platypelloid. The posterior segmelH determines the type of pelvis, whereas the anterior segment determines the tendency. These are both determined because many pelves are nor pure but are mixed types. For example, a gynecoid pelvis with an android tendency means that rhe posterior pelvis is gynecoid and the anterior pelvis is android shaped.
From viewing the four basic types in 2-17I, the coniguration of the gynecoid pelvis would intuitively seem suited for delivery of most fetuses. Indeed, Caldwell (I 939) reported [hac rhe gynecoid pelvis was found in almost half of women.
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