The Normal Pelvis

When the primitive human evolved from walking on all fours to an upright position, changes occurred in the spinal column and the pelvis.  The spine became curved and the pelvis tilted, giving a curve to the birth canal. Thus making the female human unusual amongst mammals in that the passage taken by the fetus is not straight  (Naaktgeboren 1989 cited in Silverton 1993). Caldwell and Molloy (1940) cited in Silverton (1993) classified four types of female pelves which were appropriate for the vast majority of women.The most frequent of these being the Gynaecoid pelvis which is regarded as the true female pelvis and is present in approximately 50% of women who have a pure pelvis and 50% of those with a pelvic variant (Silverton 1993).

The primary function of the pelvis is to allow movement of the body, especially walking, running, sitting and kneeling. A woman's pelvis is adapted for childbearing in that in comparison to the male pelvis the brim is rounder and wider. The pelvis contains and protects the reproductive organs as well as the bladder and rectum. For a midwife it is important to have a working knowledge of the pelvic anatomy.

The pelvis is comprised of four bones Two innominate bones: Each of these bones is made up of three parts: the ilium, the ischium and the pubis.

The iliumis the large flared out part. Its concave inner surface is the iliac fossa. The curved upper border is the iliac crest. When you place your hand on your hip it rests on the iliac crest. At the front of the iliac crest there is a bony prominence known as the anterior superior iliac spine and below it is the anterior inferior iliac spine. At the other end of the iliac crest are two similar points called the posterior superior and posterior inferior iliac spines.

The ischium forms part of the acetabulum above whilst the thick lower part is the ischial tuberosity. When you are sitting down you are sitting on your two ischial tuberosoities. These can be palpated through the buttocks and the distance apart can be assessed by placing a closed fist between them.The slight projection behind and just above the tuberosity is called the ischial spine.The two ischial spines can be palapated vaginally and in labour the station of the fetal head is estimated in relation to them.

The pubis is a small bone that has a body and two projections called, the superior ramus and the inferior ramus. The two pubic bones meet at the symphysis pubis. The two inferior rami form the apex of the pubic arch, merging into a similar ramus on the ischium, this forms the anterior boundary of the obturator foramen and the subpubic arch. In the normal gynaecoid pelvis the subpubic arch should be at least 90°.

Sacrum: The sacrum is the wedge-shaped bone consisting of five fused vertebrae, the first of which has a prominent upper border known as the sacral promontory. This is an important pelvic landmark. It projects forward decreasing the anterposterior diameter of the pelvic brim. If this diameter is seriously decreased it can impair the descent of the fetal head into the pelvis. The smooth concave anterior surface is referred to as the hollow of the sacrum and the areas either side are the alae or wings. The convex posterior surface is roughened to receive attachments of muscles. The sacrum is perforated by four sets of holes or foramina through which the sacral nerves pass.

Coccyx: The coccyx is at the lower part of the spine and is a vesitigal tail. It is a small triangular-shaped bone consisting of four fused vertebrae. The coccyx gives attachment to ligaments, deep muscles of the pelvic floor and to muscle fibres of the anal sphincter. During labour the coccyx moves backwards to enlarge the pelvic outlet allowing more space for the passage of the fetus.

To test your knowledge visit student activity 1.

Pelvic Joints: There are four pelvic joints. One symphysis pubis; two sacro-iliac joints; one sacrococcygeal joint.

The pelvic joints and ligaments are relaxed in pregnancy to hormonal action.This relaxation allows a slight increas in the pelvic measurements. This could be advantageous for some women during labour but for others it can increase the feeling of instatbility in pregnancy when walking and can be responsible for the low backache experienced by some women.

The symphysis pubis: A slightly movable joint formed at the junction of the two pubic bones. These are united by a pad of cartilage.

The sacroiliac joints: These are the strongest joints in the body. They are formed where the ilium joins with the first two sacral vertebrae thus connecting the spine to the pelvis.

The sacrococcygeal joint: This is a hinge joint between the sacrum and the coccyx, which, at the end of labour, allows the coccyx to be deflected backwards facilitating delivery of the fetus.

Pelvic Ligaments: The pelvic joints are reinforced by powerful ligaments to ensure strength and stablility.

Sacroiliac ligaments: These ligaments pass infront of and behind each sacroiliac joint.

Symphysis pubis: ligaments are used to bridge spaces in the walls of the pelvis.

Sacrotuberous ligaments: these two ligaments extens from the sides of the sacrum to the iliac tuberosities crossing the greater and lesser sciatic notches.

Sacrospinous ligaments: these pass from the sides of the sacrum to the ischial spines, extending across the greater sciatic notch.

An understanding of the pelvis, the bones and ligaments is important in midwifery practice to enable you to understand and appreciate some of the minor as well as major problems women may encounter during pregnancy, labour and in the postnatal period.