Chapter 5. Female pelvis in obstetrics. Fetus in labor

5.1. FEMALE PELVIS IN OBSTETRICS

Bony pelvis is a solid receptacle for internal genitals, the rectum, bladder and the surrounding tissues. The structure of pelvis is an important issue in obstetrics. The pelvis is divided into two distinct portions:

• greater pelvis (or false pelvis; pelvis major, Latin);

• true pelvis (true, or small pelvis; pelvis minor, Latin). The border between the false and true pelvis runs through:

• the upper brim of symphysis and pubic bones in front;

• the innominate lines on the sides;

• the promontory at the back.

The plane passing between the false and true pelvis is the plane of pelvic inlet (Fig. 5.1). The female true pelvis forms the birth canal, the passageway through which the fetus is expelled during parturition.

Fig. 5.1. Female pelvis

The false pelvis is considerably wider than the true pelvis; it is bounded by:

• the alae of iliac bones on the sides;

• the last lumbar vertebrae in the back;

• the lower portion of abdominal wall in front.

The capacity of the false pelvis can change depending on contraction or relaxation of muscles in the anterior abdominal wall.

The false pelvis is accessible for external examination; its dimensions can be determined quite accurately. The greater pelvis size serves as an indirect indication of the size of true pelvis, which is less accessible for immediate examination. Meanwhile it is the true pelvis dimensions that are of greatest importance as the fetus is delivered through the unyielding bony channel of the true pelvis. The true pelvis is the bony part of the birth canal:

• the posterior wall of the true pelvis consists of the sacrum and coccyx;

• the lateral walls are formed by ischial bones;