Yes the ischium and ilium are the same bone so if one end goes out the other end comes in. With an OP baby or a head coming into the pelvis crooked the pelvis can be pushed wide throughout top to bottom with a posterior sacrum that will widened PSIS’s. I teach more about this in the Treating the Postpartum Pelvis Course.
You mention PSIS splay in birth. If the ischium splay/out let opens, isn’t the inlet, near where PSIS are, actually going to compress together?
Yes the ischium and ilium are the same bone so if one end goes out the other end comes in. With an OP baby or a head coming into the pelvis crooked the pelvis can be pushed wide throughout top to bottom with a posterior sacrum that will widened PSIS’s. I teach more about this in the Treating the Postpartum Pelvis Course.