Ischium

The ischium is the thick lower part of the pelvis, formed from two fused bones — one on either side. When a woman is in labour, the descent of the fetal head as it moves down the birth canal is estimated in relation to the ischial spines, which are inward projections of the ischium on each side. The ischial spines are smaller and rounder in shape in the woman's pelvis than in that of the man.