The pelvic inlet

The pelvic inlet is formed by the pelvic brim. 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 below in both directions (top to bottom; and transverse or side to side). When you look at the figure below 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.