I encountered this term in my copy of Holistic Midwifery Volume II written by Anne Frey. It is one of the texts in my midwifery program and covers extensively (its almost 2,000 pages!) the physiology and care of labor and birth.
I remember when this happened during Belle's birth. I even commented on it when I wrote her birth story. I didn't know the name of it then so I'm pleased to learn about it now. At the time I described it as:
I felt tightness radiating down into my thighs and it felt like the bones of my pelvis and hip were being stretched outwards.I later thought that this might have something to do with the hip pain that lingered after that pregnancy. I learned that it might be SI dsyfunction (sacroiliac joint) and posted about it here. Finding out that it was an actually physiological process that is part of normal birth helped me feel that the strange sensation wasn't an injury but my body working as it was designed to.
I thought at the time that when I felt my hips spread during Belle's birth that something unnatural had happened. I still think that maybe my SI issues have something to do with this spreading. It happened so quickly--it was like my SI joints abruptly slid out as far as they could like sticky slide locks on a door. Perhaps they slid more forcefully and quickly than is typical when the sacrum moves and the back opens.
This phenomenon is only observed when upright birth is allowed. When a woman in laying on her back in bed, the sacrum is prevented from moving and the pelvis does not expand in conjunction with the fetal ejection reflex.
I don't recall feeling this happen when giving birth to my son. Not surprising, since I felt pressured into birthing on my back. The only way I felt that I had been able to reposition more comfortably as he was descending and crowing was by twisting my hips so that I was more on my side.
Someday I may find more detailed description of this phenomenon. It seems like it hasn't been described or investigated by researchers enough to know what is normal "opening of the back" and what is extreme. As it is, the term is hardly known except in some circles. Hospital birth attendants and medical researchers wouldn't see it because the vast majority of birthing women they see are on their backs. Upright/vertical birth isn't even an option in many hospital birth environments.
Have any of my readers heard of this? Care to theorize with me on the possibility of normal opening of the back and extreme opening on the back?