The roughly circular space enclosed by the pubic bones at the front, and the ischium on either side at the back, is called the pelvic canal — the bony passage through which the baby must pass. This canal has a curved shape because of the difference in size between the anterior (front) and posterior (back) borders of the space created by the pelvic bones. You can see it from the side view in Figure 6.2.
Figure 6.2 The pelvic canal seen from the side, with the body facing to the left.
6.2.1 The size and shape of the pelvis
The size and shape of the pelvis is important for labour and delivery. Well-built healthy women, who had a good diet during their childhood growth period, usually have a broad pelvis that is well adapted for childbirth. It has a round pelvic brim and short, blunt ischial spines. (Doctors and midwives call this shape a ‘gynaecoid’ pelvis.) It gives the least difficulty during childbirth, provided the fetus is a normal size and the birth canal has no abnormal tissue growth causing an obstruction.
There is considerable variation in pelvis shapes, some of which create problems in labour and delivery. A narrow pelvis can make it difficult for the baby to pass through the pelvic canal. A deficiency of important minerals like iodine in the diet during childhood may result in abnormal development of the pelvic bones. Stunting (being much shorter than average for age) due to malnutrition and/or infectious diseases can also result in a narrow pelvis.
Next, we look at the shape of the pelvic canal in more detail, and distinguish between the pelvic inlet (the roughly circular space where the baby’s head enters the pelvis — Figure 6.3), and the pelvic outlet (the roughly circular space where the baby’s head emerges from the pelvis. As you will see in the next section, the inlet and the outlet of the pelvis are not the same size.
6.2.2 The pelvic inlet
The pelvic inlet is formed by the pelvic brim, which you saw in Figure 6.1. The pelvic brim is rounded, except where the sacral promontory and the ischial spines project into it. The dimensions in centimetres (cm) of the pelvic inlet are shown in Figure 6.3 in both directions (top to bottom; and transverse or side to side). When you look at Figure 6.3, imagine that you are a baby in the head-down position, looking down on the pelvis from above, at the space you must squeeze through! It is just 13 cm wide (on average) and 12 cm from top to bottom.
Figure 6.3 Diameters of the pelvic inlet, viewed from above.
The pelvic outlet is formed by the lower border of the pubic bones at the front, and the lower border of the sacrum at the back. The ischial spines point into this space on both sides. Figure 6.4 shows the dimensions of the space that the fetus must pass through as it emerges from the mother’s pelvis. As you look at Figure 6.4, imagine that you are the birth attendant who is looking up the birth canal, waiting to see the fetal head emerging.
Figure 6.4 Diameters of the pelvic outlet, viewed from below.
What do you notice when you compare the dimensions of the pelvic inlet (Figure 6.3) and the pelvic outlet (Figure 6.4)? Which is the narrowest?
The narrowest diameter for the fetus to pass through is the pelvic outlet, which is only 11 cm wide in the average female pelvis.
End of answer
It is difficult to see from Figures 6.3 and 6.4, but the fetus has to rotate in order to get through the pelvic canal. This is because the pelvic inlet is 13 cm wide, whereas the pelvic outlet is only 11 cm wide. In order to fit through the pelvic outlet at its widest dimension (12.5 cm from top to bottom), the fetus must rotate so it ‘presents’ its head to the widest dimension of the pelvic cavity at every point as it passes through. The largest part of the fetus is the skull, so the baby’s head rotates first, and the shoulders and the rest of the body follow. You will learn all about this in the Labour and Delivery Care Module. First, we have to look more closely at the structure of the fetal skull.