I have personal and professional experience of this happening. I know myself that it was more than just two different people doing a vaginal exam coming out with two different diagnosis. When I first experienced this, I was in labour and transferred from home to hospital as my daughter was lying in a transverse position. Later as a student I had heard of situations where on a change of shift, a woman would appear to go back on her dilation when a new midwife took over her care. I was once told off for making a woman 8cm when she was made 4 by the new midwife who took on her care. Being a student, I apologised, but had no doubt in the exam I had done and knew in my heart that this woman had progressed through the night with myself and my mentor to 8cm or possibly more.
I believed in the sphincter law but could not say this to the midwife who told me off, because this is not taught in university although to me it is common sense. Please watch this video of Ina May’s clear and reasonable explanation.
http://www.youtube.com/watch?v=JQn_nTia7FY
http://www.youtube.com/watch?v=dCfSZn28FgM
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The Barcelona Midwife 