CHAPTER 11 The female pelvis is generally wider and broader than that of males, with a more pronounced oval‐shaped inlet (Figure 11.1). This ‘gynaecoid’ pelvis is ultimately different to accommodate the process of childbirth. In obstetric terms, the female pelvis is considered as two regions: the superior section is referred to as the greater or ‘false’ pelvis. It has limited obstetric relevance but provides support for the lower abdominal viscera. Intuitively, the inferior section is known as the lesser or ‘true’ pelvis, which contains the pelvic cavity and pelvic viscera. The junction between the greater and lesser pelvis is known as the pelvic inlet. The outer bony edges of the pelvic inlet are called the pelvic brim. This is the superior margin of the pelvic cavity and is bounded by the sacral promontory posteriorly, laterally by the iliopectineal lines, and anteriorly by the symphysis pubis. The pelvic inlet determines the size and shape of the birth canal. Significantly, the transverse diameter tends to be greater than the anteroposterior (AP) diameter. The pelvic cavity has a curved shape because of the difference in size between the anterior and posterior borders of the space created by the pelvic bones. Roughly circular in shape with the transverse and AP diameters tending to be similar, it is bounded by the sacrum posteriorly, laterally by the pubic bone and the obturator fascia and the inner aspect of the ischial bone, and anteriorly by the symphysis pubis. The cavity is referred to as the pelvic canal and is the bony passage through which the baby must pass. This is the narrowest bony part of the pelvis for the baby to pass through and marks the inferior margin of the pelvic cavity. It is bounded posteriorly by the coccyx, laterally by the ischial tuberosities, and anteriorly by the pubic arch. In regard to the mechanism of normal delivery it is important to note that now the AP diameter tends to be greater than the transverse diameter. Figure 11.2 identifies of certain regions and landmarks of the fetal skull. These have particular importance for obstetric care because they may indicate the presenting part of the fetus. Additionally, the following terms are also used as reference points: the sinciput refers to the front of the head whilst the occiput refers to the back of the head and the mentum refers to the chin. The fetal skull bones are as follows: Sutures are joints between the bones of the skull that permit gliding of one bone over another during moulding of the head. In a vertex presentation the diameter of the head can be reduced to allow easier passage through the birth canal. The sutures begin to harden during childhood, inhibiting movement. This continues into early adulthood until the growing process completes. There are two key landmarks created where the sutures meet, and these are referred to as the fontanelles: The process of labour is normally divided into three stages as this makes it easier to describe the progress a mother is making. These are explained below. A very fast labour with a duration of less than 1 hour is referred to as a ‘precipitate labour’. The amniotic membranes and fluid surround the fetus, protecting it within a sterile environment against ascending bacterial infection. The first stage can be further divided into the ‘latent’ and the ‘active’ phases. This phase of labour can be experienced for several weeks prior to the onset of active labour. Uterine contractions experienced during this phase cause the cervix to thin and soften before it can start to dilate. Contractions are often irregular with a stop/start pattern without becoming stronger or longer. This is normal.
Normal labour and delivery
11.1 Normal labour and delivery
Anatomy of the female pelvis
Pelvic inlet
Pelvic cavity
Pelvic outlet
11.2 Fetal skull
11.3 Stages of labour
11.4 First stage of labour
Latent phase of labour
Active phase of labour