As described in Ina May’s Guide to Childbirth, the vaginal and cervical sphincters operate just as the excretory ones do. “Each sphincter’s job is to relax and expand so that it can open comfortably and wide enough to allow the passage of whatever must move through” .
Since the cervix and vagina are sphincters, they operate under the Sphincter Law. This means that they do not respond to orders (such as someone telling a mother to push), they work best “in an atmosphere of familiarity and privacy”, they “may suddenly close when their owner is startled or frightened”, and they are more easily opened when the mouth and jaw are relaxed.
Furthermore, “the presence of a strange person in the birth room, especially if the person is male and not an intimate companion of the laboring woman, frequently (although not always) slows or stops labor” .
The atmosphere while birthing at home naturally works well with the Sphincter Law. The mother is comfortable with her surroundings, and the only people present are those that she knows. She is less likely to have fear as her midwives will generally treat labor and birth as a natural occurrence rather than as a medical situation that must be managed.
Since the mother will be comfortable and have less fear, she may experience labor as a more joyous process, possibly even smiling and laughing which would further allow her body to work with the Sphincter Law. I certainly would not have felt as comfortable birthing in a hospital as I was at home, and the Sphincter Law shows that the mother’s comfort level is intimately connected to how smoothly her labor can progress.
 Ina May’s Guide to Childbirth, Ina May Gaskin, 2003, pp. 141 and 170-179.