Thinning of the cervix that occurs in preparation for childbirth. The cervical canal shortens from 2cm to paper thin edges and the canal itself disappears. Only the external opening of the uterus remains of what was the cervix. The edges of the internal opening of the cervix are drawn upward several centimeters to become part of the lower uterine segment. When the cervix becomes as thin as the adjacent lower uterine segment, it is considered to be completely effaced.