The pelvis is divided into true & false pelvis which are separated by pelvic brim.
The plane of pelvic brim is bounded in front by the symphysis pubis [the joint separating the two pubic bones] on each side by the upper margin of pubic bone, iliopectineal line & ala of sacrum. Posteriorly by promontary of the sacrum.
The anteroposterior diameter of brim [true conjugate] is 11cm & the transverse diameter is 13.5cm.
Can be described as an area bounded in front by the middle of symphysis pubis ,on each side by pubic bone, obturator fascia & inner aspect of ischial bone & spines. Posteriorly bounded by the junction of second & third pieces of sacrum.
The cavity is roomy circular with anterioposterior & transverse diameters both measure 12cm.
THE PELVIC OUTLET
Is roughly diamond shaped & bounded infront by the lower margin of symphysis pubis, on each side by the descending ramus of pubic bone , ischial tuberosity & sacrotuberous ligament & posteriorly by the last piece of the sacrum.
In gynaecoid pelvis the subpubic arch is wide & tuberosities are far apart. The anterior-posterior diameter is 13.5cm & the transverse diameter is 11cm.
It forms part of birth canal, it is formed by the two levator ani muscles which with their fascia form amusculofascial gutter during the second stage of labour with the opening of the vagina looking forward between sides of the gutter
THE PELVIC AXIS
Is an imaginary curve line shows the path which the centre of fetal head follows during its passage through the pelvis, it is obtained by taking several anteroposterior diameters of the pelvis & joining their centers.