During the process of labour the cervix(mouth of the womb) thins out and dilates(opens up) to allow the passage of the baby from the womb to the birth canal. However, sometimes this shortening of the vaginal portion of the cervix, thinning of the walls and dilation of the cervix is not caused by labour, but by structural weakness in the cervix itself. This is called cervical incompetence.
Attribution: Avallain