Sonographic imaging of the cervix across gestation has enhanced our understanding of cervical performance during pregnancy. Cervical effacement is one of the first steps in the parturition process, preceding labor by at least four to eight weeks. Effacement begins at the internal cervical os and proceeds caudally in a sequential pattern, thus it can be seen by ultrasound, but is not always appreciated by digital or visual examination. As cervical length decreases in the second trimester, the risk of spontaneous preterm birth increases [1-5], especially when effacement occurs early in the second trimester .
Although a short cervix can be viewed as evidence that the process of parturition has begun, the cause of preterm cervical shortening is often unclear. It has been attributed to biological variation, occult uterine activity, uterine overdistention, congenital or acquired cervical insufficiency, decidual hemorrhage, and infection or inflammation. Risk factors for short cervix and cervical insufficiency are discussed separately. (See "Cervical insufficiency".)
Transvaginal ultrasound (TVU) examination is the most reproducible and reliable technique for cervical assessment . When a short cervix is suspected by transabdominal sonography before 25 weeks, TVU examination should be performed to obtain the best estimate of cervical length. Transabdominal images of the cervix are less reproducible; thus, they should not be used for clinical management [8-10].
Transperineal sonography is more difficult, less reproducible and less valid than transvaginal sonography [11,12].
Timing — Cervical length measurements before 14 weeks of gestation have limited clinical value [13,14]. However, in some particularly high-risk pregnancies, such as those with prior second trimester losses and/or large (or multiple) cone biopsies, cervical shortening has been seen as early as 10 to 13 weeks of gestation and is abnormal and associated with a high risk of second trimester loss . Reproducible measurement of cervical length becomes possible at about 14 weeks of gestation, when the cervix normally becomes distinct from the lower uterine segment.