Ultrasound Evaluations of Cervical Length in Preterm Labor

Abstract & Commentary

Synopsis: In women with threatened preterm labor, sonographic measurement of cervical length helps distinguish between true and false labor.

Source: Tsoi E, et al. Ultrasound Obstet Gynecol. 2003;21(6):552-555.

I realize that many of my latest OB/GYN Clinical Alert submissions have had a heavy flavoring of ultrasound, and I had promised myself to head in another direction this month. However, unfortunately (or, I hope, fortunately) a paper popped up in Ultrasound in Obstetrics and Gynecology that I feel is of such value that I should pass it on immediately.

Tsoi and colleagues studied 216 patients presenting with regular painful contractions between 24 and 36 weeks. A simple transvaginal ultrasound evaluation of cervical length (CL) was accomplished on admission. Management of these patients was based on the desires of the responsible physicians who were blinded to the results of the CL exams.

Forty-three patients had cervices of less than 1.5 cm and 16/43 (37%) delivered within 7 days of admission. Of those with short cervices, delivery occurred within 7 days in 6/14 (42%) on tocolytics and 10/29 (35%) who were managed expectantly.

Most importantly, only one of 132 patients with CL of more than or equal to 1.5 cm delivered within 7 days, and she was on tocolytics.

Comment by John C. Hobbins, MD

Threatened preterm labor has remained one of the most vexing unsolved problems in obstetrics. Recent investigation has been directed toward identifying the patients most benefiting from current methods of tocolysis.1,2 On one hand, randomized clinical trials have failed to show a benefit to most tocolytics tested. On the other hand, most patients enrolled in these trials were undoubtedly not in labor to start with, and perhaps the tocolytics tested were not really given a fair chance to succeed.

As pointed out in a previous OB/GYN Clinical Alert, fetal fibronectin has the potential to separate true from false labor, and the beauty of the test is in its high negative predictive value, which exceeds 99%.3 The only drawbacks are its expense, the short delay for the results to return, the logistics of sampling before a digital exam is done, and, frankly, the inability of clinicians to trust the results enough to lay off the use of tocolytics, with all their inherent morbidity, when a negative result comes back.

Now we have one study that strongly suggests that a simple ultrasound evaluation of CL can identify the patient with a very low likelihood of delivering within 1 week of the examination, while allowing us to concentrate only on the ones that are at greatest risk of early delivery.

Our policy will now be to send the fetal fibronectin to the laboratory when the cervix is less than 1.5 cm to see if the results will affect the predictive value in those at greatest risk. It is also hoped that investigations will cone down on those with short cervices when testing the worth of current tocolytics through randomized trials.

Dr. Hobbins is Professor and Chief of Obstetrics, University of Colorado Health Sciences Center, Denver.


1. Bergelin I, Valentin L. Ultrasound Obstet Gynecol. 2003;21:556-563.

2. Pardo J, et al. Ultrasound Obstet Gynecol. 2003;21: 464-466.

3. Hobbins JC. OB/GYN Clinical Alert. 2001;18(5):33-35.