5. Identification of mid-line sagittal plane of the cervix and looking in the proximal one third of the image for the internal cervical os then pulling back the probe until the lightest touch provides good image of the cervical canal and moving the probe slightly to get the best long axis of the cervix then measuring the cervical length three times by placing the calibers appropriately and recording the distance between internal and external os then at last recording the measurement of the best image .