In normal labour we studied that occiput comes out first

in normal labour we studied that occiput comes out first and for that it undergoes internal rotation so that it comes to lie behind the pubic symphysis so that it lies in anteroposterior diameter of pelvis which is the largest diameter at the pelvic outlet.

But mam my doubt is that why in occipitoposterior position head rotates through a long circle to come in occipitoanterior position because even in occipitoposterior position also it is still in anteroposterior diameter only?

Thank u mam

Because unless it rotates it lies in AP diam of inlet which is shorter than transverse diam in gynecoid pelvis … at the level of outlet AP is longer than transverse diam