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.