You can see the diameters of the pelvic inlet and outlet if you look back to Figures 6.3 and 6.4 in the Antenatal Care Module, Part 1.
As the head reaches the pelvic floor, it typically rotates to accommodate the change in diameters of the pelvis (Figure 1.3, diagram 3). At the pelvic inlet, the diameter of the pelvis is widest from right to left. At the pelvic outlet, the diameter is widest from front to back. So the baby must rotate from lying sideways to turning its face towards the mother's backbone (Figure 1.3, diagram 4). When the rotation is complete, the back of the baby's head is against the front of the mother's pelvis). The sagittal suture in the fetal skull is no longer at an angle, but points straight down towards the mother's backbone. This movement is called internal rotation because it occurs while the baby is still completely inside the mother.