In obsteric and gynaecological imaging, the cervical length (CL) is defined as the length between the internal cervical os and the external cervical os. the cervical length is most accurately assessed on a transvaginal scan with an empty bladder. in a normal gravid situation the cervical length is considered to be at least 30 mm (3 cm) the presence of a full bladder can increase the cervical length particularly on a trans-abdominal scan.