One of the doulas on a mailing list that I'm on recently posted a question about whether it's possible for a woman's cervix to reverse in dilation and for the past few days the responses have been pouring in. Emphatically, yes. Many of the women had either been with a mother whose cervix reversed, or observed it themselves, or experienced it during their births. In Ina May Gaskin's Spiritual Midwifery she tells the story of one of the women who gave birth on The Farm who, afraid that labor was moving too quickly and was getting out of control, undilated.
"Judith...on hearing that she was fully dilated, became very sober, very serious. If someone said something funny, she was the only one who didn't laugh...She coughed once, a shallow, polite little cough that obviously didn't get anything done. By this time I was getting curious as to what was going on...So I put on another sterile glove and once again checked Judith's dilation. She was only four centimeters dilated! I was amazed. I had never known before that a woman could go backwards and undilate herself. When Judith heard what she had done, she admitted that she had been worried that the labor had been coming on so fast that it was getting out of control. I told her that it was supposed to feel like that...She relaxed and in one or two more rushes [contractions] she was fully dilated again and after a few good pushes, Abigail was born." (201-202)
She also tells the story here (around 3:00).
Ina May Gaskin repeats quite often that a "loose mouth makes a loose bottom," and that an upset or scared birthing mother will have a hard time delivering, one of the reasons why dilation reversals and stalled labor are occasionally observed when women shift into the unfamiliar surroundings of the hospital. Ina May points to the ease with which most relaxed, comfortable, good-humored and confident mothers have in birthing, which is made easier by a comfortable environment.
There are some medical explanations as well including swelling and membrane rupture here, which are less likely to consider the mother's emotional state as a contributing factor. Also, cervix measurements aren't done with great accuracy, so different nurses over time might take different readings based on their individual perceptions, which might account for minor "reversals." Overall, however, I found very little online to explain what's going on with dilation reversals, something which Ina May also talks about in the above video, but stress does make the body do all sorts of interesting things and so I don't doubt her expert opinion.