Prediction of preterm birth: cervical sonography.

The cervix has to open to allow vaginal birth. Ultrasound has now shown that this lower part of the uterus begins to show changes weeks before eventual birth. Only transvaginal ultrasound should be used to evaluate the cervix for prediction of preterm birth (PTB). The shortest best cervical length (CL) is the most effective measurement for clinical use. Proper technique is paramount for accurate results. The risk of PTB increases with ever shorter CL (<25 mm). Other factors that must be carefully considered when using CL for prediction of PTB are number of fetuses, risk factors for PTB, and gestational age at screening.