Joints and ligaments of the pelvis

The joints of the pelvis connect the innominate bones at the pubis anteriorly and to the sacrum posteriorly, and the sacrum to the coccyx (see Fig. 24.2). These joints are cartilaginous in type, consisting of plates of fibrocartilage. The pelvis also provides attachment points for ligaments, which are bands of tissue connecting two structures. In normal circumstances, ligaments do not possess the ability to stretch, and therefore prevent excessive movements within the joints, enhancing stability.

Figure 24.2 The ligaments and joints of the pelvis. A. Superior view. B. Sagittal section.

In pregnancy, the hormones relaxin, progesterone and oestrogen affect the joints and ligaments, enabling some movement of the joints to facilitate birth. Pelvic pain sometimes occurs during pregnancy, birth or post partum and is thought to be linked to overstretching of ligaments in the pelvis and lower spine (Rost et al 2004).

The true pelvis

The true pelvis, through which a baby negotiates passage during labour and birth, is the most significant part of the pelvis. This is divided into three regions, known as the brim, cavity and outlet (see Fig. 24.3). As the presenting part descends into the pelvis, the baby negotiates each aspect of the true pelvis simultaneously. For example, in a cephalic presentation, as the baby’s head crowns, the presenting part negotiates the outlet, most of the baby’s head is in the cavity and the shoulders are at the brim.

Figure 24.3 The axis of the true pelvis.

It is important to appreciate that the pelvis is three-dimensional. The pelvic measurements of the brim, cavity and outlet are viewed through a cross-section, whereas measurements incorporated in the pelvic conjugates are viewed through a sagittal section. These two measurements inform a pelvic assessment.

Pelvic measurements (Fig. 24.4)

The pelvic brim is the inlet to the true pelvis and is almost circular, except posteriorly, where the sacral promontory juts into the brim (see Fig. 24.5).

Figure 24.4 Pelvic measurements.

Figure 24.5 Superior view of the pelvis to show the landmarks of the pelvic brim.

The landmarks of the pelvic brim describe the interplay between the fetus and the pelvis as the presenting part descends, and are a fundamental part of the assessment of descent and engagement of the presenting part. It is not the components of the brim that are important, it is the part that the brim plays as a whole in the assessment of progress during pregnancy and labour. This is the first test that the fetus has to pass as it descends through the pelvis. The midwife assesses engagement of the presenting part during abdominal and vaginal examinations (see Chs 35–37).

The pelvic cavity extends from the brim to the outlet of the pelvis. In the anteroposterior view the cavity is wedge shaped: shallow at the front and deep at the back. Viewed from above it is circular in shape in a gynaecoid pelvis and designed to facilitate the descent and rotation of the presenting part. The boundaries of the cavity are:

The pelvic outlet is diamond shaped and partly bound by ligaments. It can be described in two ways:

Figure 24.6 The relationship of the pelvic conjugates and the fetal negotiation of the conjugates.

The anatomical boundaries for the outlet of the pelvis are:

The obstetric outlet is bounded by:

Pelvic conjugates

A conjugate is a measurement taken from one point in the pelvis to another. In midwifery there are the anatomical, obstetrical and internal (diagonal) conjugates (Fig. 24.6).

Angles and planes

These are mathematical concepts applied to the pelvis. When standing, the pelvis slopes into a position where the pubis is lower than the sacral promontory – described as an angle of 55° to the horizontal or to the floor. This slope continues through the cavity, reducing its angle to 15° at the outlet. The fetal head must negotiate the curve created by the changing angles within the pelvis as it enters the pelvic brim in a downward and backward direction. It emerges from the outlet in a downward and forward direction as the presenting part reaches the pelvic floor. The curve created in the pelvis is known as the curve of Carus.

The term plane describes the relationship between the pelvis and a flat surface, such as the floor, highlighting the tilt of the pelvis in a normal female skeleton. Hypothetical angles are then created in relation to the degree of tilt of a particular individual (see Figs 24.7 and 24.8), which provide a representation of the angles in relation to the planes of the pelvis. Figure 24.8 shows the axis (curve of Carus), an imaginary line through which a fetus rotates as it passes through the pelvis.

Figure 24.7 The pelvis, showing the degrees of inclination: inclination of the pelvic brim to the horizontal, 55°; inclination of pelvic outlet to the horizontal, 15°; angle of pelvic inclination, 135°; inclination of the sacrum, 90°.

Figure 24.8 The axis of the birth canal in upright position.

In an abnormal pelvis, the plane of the pelvis may be significantly altered, affecting the axis of the birth canal and consequently the direction of the fetus through the pelvis. The midwife needs to consider the axis of the birth canal when women adopt alternative positions for childbirth during labour and delivery.

The subpubic angle is between the two inferior pubic rami forming the pubic arch (Fig. 24.1). In a gynaecoid pelvis, this should be approximately 90°, enabling two finger widths to sit in the apex of the pubic arch during vaginal pelvic assessment.

Pelvic variations

Although there are four recognized pelvic categories (Caldwell et al 1940) (Table 24.1), variations within these categories can occur. Some women may have mixed features, such as a gynaecoid posterior pelvis and android forepelvis. The most important factor is the true pelvic space available for the fetus to descend and emerge from the pelvis. The pelvic size and shape cannot be viewed in isolation from other factors, such as position and size of fetus and processes of labour.

Table 24.1 Pelvic categories

Other factors that may influence the size and shape of the pelvis include:

Other pelvic types identified

Any injury or disease of pelvic bones may significantly affect the dimensions of the pelvis, impacting on the outcome of labour and birth. Table 24.2 outlines the classification and characteristics of unusual pelves, each of which may have a mixture of characteristics, with the shape depending upon the degree and timing of damage. It is important that the midwife assesses women at risk of pelvic dysfunction as early as possible antenatally.

Table 24.2 Classified unusual pelves

Figure 24.9 Posterior asynclitism.

Pelvic assessment

Pelvic assessment enables estimation of whether the fetus will successfully pass through the pelvis during labour and delivery, by assessing the pelvic size and outlet. Although this can be undertaken at any time before or during pregnancy, the relationship of the pelvis to the fetal skull can only be assessed from 37 weeks’ gestation (antenatally or during labour). Pelvic assessment is no determinant of outcome but contributes to the overall assessment of pelvic adequacy.

The assessment must include:

• vaginal examination – determining the size and shape of the pelvis by assessing the following:

prominence of sacral promontory (usually cannot be palpated on vaginal examination)

prominence of ischial spines and, if identified, the distance between them

angle of pubic arch (usually accommodates two finger widths at the apex of the arch)

prominence of ischial tuberosities (usually accommodates four knuckle widths when measured externally at the level of the perineum).

It may also include:

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