Obstetrics and Gynecology - A.M. Hromova 2000

The Female Pelvis. The Pelvis from an Obstetric Perspective

The bony pelvis consists of two hip bones, the sacrum, and the coccyx, which are firmly connected by cartilaginous interlayers and ligaments (Fig. 12). The hip bone is formed by the fusion of three bones: the ilium, the ischium, and the pubis. They connect in the region of the acetabulum.

The sacrum consists of 5–6 immovably connected vertebrae that fuse into a single bone.

The coccyx consists of 4–5 rudimentary vertebrae.

The upper part of the bony pelvis is open anteriorly; this section is called the false pelvis (greater pelvis). The lower part is a closed bony Structure known as the true pelvis (lesser pelvis). The boundary between the false and true pelvis is the terminal line (linea terminalis): anteriorly, it is formed by the upper margin of the Pubic Symphysis and pubic bones; laterally, by the arcuate lines of the iliac bones; and posteriorly, by the sacral promontory. The plane between the false and true pelvis constitutes the pelvic inlet. The false pelvis is significantly wider than the true pelvis; it is bounded laterally by the iliac wings, posteriorly by the lower lumbar vertebrae, and anteriorly by the lower portion of the anterior abdominal wall.

All women undergo measurements of the false pelvis (Fig. 13). A correlation exists between the dimensions of the false and true pelvis. By measuring the false pelvis, we can draw Conclusions about the dimensions of the true pelvis.

Normal pelvic dimensions:

distantia spinarum - the distance between the anterior superior iliac spines - 25-26 cm;

distantia cristarum - the distance between the outermost points of the iliac crests - 28-29 cm;

distantia trochanterica - the distance between the greater trochanters of the femurs with the legs brought together - 31-32 cm;

conjugata externa - (external conjugate) - the distance from the midpoint of the upper margin of the symphysis to the upper angle of the Michaelis rhombus (measured with the woman lying on her side) - 20-21 cm (Fig. 14).

The Michaelis rhombus is a diamond-shaped depression in the sacral region bounded by: superiorly, the indentation below the spinous process of the fifth lumbar vertebra (suprasacral fossa); inferiorly, the points corresponding to the posterior superior iliac spines. The vertical length of the rhombus is typically 11 cm, and its transverse width is 10 cm.

The diagonal conjugate - the distance from the lower margin of the symphysis to the most prominent point of the sacral promontory, determined during a vaginal examination. With normal pelvic dimensions, it is 12.5-13 cm (Fig. 15).

The true conjugate size (the anteroposterior diameter of the pelvic inlet) is determined by subtracting 9 cm from the length of the external conjugate or subtracting 1.5-2 cm from the length of the diagonal conjugate (depending on Solovjev's index).

Solovjev's index is the wrist circumference divided by 10. This index provides an estimation of the woman's bone thickness. The thinner the bones (index = 1.4-1.6), the greater the capacity of the true pelvis. In such cases, 1.5 cm is subtracted from the diagonal conjugate to obtain the true conjugate length. If Solovjev's index is 1.7-1.8, 2 cm is subtracted.

The pelvic inclination angle - the angle between the plane of the pelvic inlet and the horizontal plane - is 55-60° (Fig. 16). Deviations in either direction can negatively affect the course of labor.

The height of the symphysis normally measures 4 cm and is assessed using the index finger during a vaginal examination.

The pubic angle - with normal pelvic dimensions, is 90-100°.

The lateral conjugate - measured from the anterior-superior iliac spine to the posterior-superior iliac spine - equals 14.5-15 cm.

The true pelvis is the bony portion of the birth canal. The posterior wall of the true pelvis is formed by the sacrum and coccyx, the lateral walls by the ischial bones, and the anterior wall by the pubic bones and symphysis. The true pelvis comprises the following divisions: the inlet, the cavity, and the outlet.

Within the pelvic cavity, a wide and a narrow part are distinguished. Accordingly, four planes of the true pelvis are defined (Fig. 16):

1 - The plane of the pelvic inlet.

2 - The plane of the pelvic cavity's wide part.

3 - The plane of the pelvic cavity's narrow part.

4 - The plane of the pelvic outlet.

The plane of the pelvic inlet passes through the upper inner margin of the pubic arch, the terminal lines, and the tip of the sacral promontory. The following dimensions are distinguished within the inlet plane:

1. The anteroposterior (direct) diameter is the distance from the sacral promontory to the most prominent point on the upper inner surface of the symphysis; this is the obstetric, or true, conjugate, which measures 11 cm.

2. The transverse diameter is the distance between the most distant points of the linea terminalis (arcuate lines), measuring 13–13.5 cm.

3. The two oblique diameters extend from the sacroiliac joint on one side to the iliopubic eminence on the opposite side of the pelvis. They measure 12–12.5 cm.

The plane of the greatest pelvic dimension (wide cavity) passes through the midpoint of the inner surface of the pubic arch, laterally through the midpoints of the acetabula, and posteriorly through the junction between the second and third sacral vertebrae.

The plane of the wide pelvic cavity features the following dimensions:

1. The anteroposterior diameter extends from the midpoint of the inner surface of the pubic arch to the junction between the II and III sacral vertebrae. It measures 12.5 cm.

2. The transverse diameter passes between the midpoints of the acetabula. It measures 12.5 cm.

The plane of the least pelvic dimension (narrow cavity) extends anteriorly through the inferior margin of the pubic symphysis, laterally through the ischial spines, and posteriorly through the Sacrococcygeal joint.

The plane of the narrow pelvic cavity features the following dimensions:

1. The anteroposterior diameter extends from the inferior margin of the symphysis to the sacrococcygeal joint. It measures 11.5 cm.

2. The transverse diameter is measured between the most distant points on the inner surfaces of the ischial spines. It measures 10.5 cm.

The plane of the pelvic outlet extends anteriorly through the inferior margin of the symphysis, laterally through the apices of the ischial tuberosities, and posteriorly through the tip of the coccyx.

The pelvic outlet features the following dimensions:

1. The anteroposterior diameter extends from the tip of the coccyx to the inferior margin of the symphysis. It measures 9.5 cm, but increases by 1.5–2 cm during fetal descent through the pelvis due to the deflection of the coccyx by the fetal presenting part.

2. The transverse diameter is measured between the most distant points of the inner surfaces of the ischial tuberosities; it measures 11 cm.

The line connecting the midpoints of the anteroposterior diameters of all pelvic planes is called the axis of the pelvis, which curves forward. It is along this line that the leading point travels through the birth canal.

Key differences between the female and male pelvis:

- the BONES OF THE female pelvis are relatively thinner and smoother;

- the female pelvis is comparatively wider, shallower, and has a greater volume;

- the iliac wings in females are more flared, making the transverse diameters of the female pelvis larger than those of the male;

- the pelvic inlet in females is transversely oval, whereas in males it is Heart-shaped;

- the pelvic inlet is larger in females, and the pelvic cavity does not taper inferiorly in a funnel-like shape as it does in males;

- the subpubic angle is obtuse in females (90–100°), but acute in males (70–75°);

- the pelvic inclination angle is greater in females (55–60°) than in males (45°).

Review Questions

1. Which bones make up the female pelvis?

2. Structural Features of the female pelvis compared to the male pelvis.

3. Why is it necessary to measure the external dimensions of the pelvis?

4. Name the external dimensions of the pelvis.

5. What planes are defined in the lesser pelvis, and what are their boundaries and dimensions?