Page 20 - 2023 Elctronic Book (3) Normal Labor_Neat

Figure 5 Diameters of the pelvic outlet, viewed from below.

It is difficult to see from Figures 3 and 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 fetus’s head rotates first, and the shoulders
and the rest of the body follow.

 Diameter of pelvic outlet:

• Antero - posterior diameters:

o Anatomical antero-posterior diameter =11cm

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