Preterm birth remains a major cause of perinatal morbidity and mortality. A short cervix is strongly associated with spontaneous preterm birth. Professional organizations support cervical length screening for singleton gestations with a prior spontaneous preterm birth and second-trimester cervical length measurements between 16-24 weeks. All interventions used to decrease the risk of preterm birth in women with a short cervix are based on clinical trials that used transvaginal cervical length measurement, but transabdominal ultrasound has been shown to correlate well with transvaginal measurement in some observational studies. Transvaginal cervical length measurement is more accurate and more reliably obtained than the transabdominal approach. Conversely, transabdominal ultrasound could have the advantage of ease of implementation and, in general, is perceived by patients to be associated with less discomfort. Currently, there is no randomized clinical study that compares head-to-head the effectiveness of transvaginal vs transabdominal ultrasound for preterm birth risk screening. This point/counterpoint article summarizes the pros and cons of the 2 ultrasound approaches and debates whether transvaginal ultrasound should be used exclusively or if transabdominal ultrasound can be incorporated in cervical length screening for prevention of preterm birth.
The Issue
Sonographic cervical length assessment to detect shortening has been shown to be an effective screening test for prediction and prevention of spontaneous preterm birth, and variations of the strategy have been widely adopted for clinical practice. Interventions used to decrease the risk of preterm birth in women with a short cervix are based on study designs that employed cervical length measurement using transvaginal ultrasound. Transabdominal ultrasound cervical length measurement appears to correlate with transvaginal measurements and has also been used for screening. However, there are limited data on implementing a screening program. This debate addresses the topic of which should be the preferred ultrasound approach–transvaginal or transabdominal–in screening for patients at high risk for spontaneous preterm birth.
1. CL assessment is unreliable with TAU
TAU does not have sufficient reliability or validity, because: (a) the bladder often needs to be adequately filled at the time of the TAU to obtain an interpretable image, resulting in elongation of the cervix and masking of any funneling of the internal os; (b) fetal parts can obscure the cervix, especially >20 weeks gestation; (c) the relatively long distance from the probe to the cervix results in lower image quality compared to TVU; (d) patient obesity can result in poor imaging; and (e) excessive manual pressure may falsely elongate and distort CL. These factors undermine the reliability and validity of results derived by TAU.
1. CL assessment is unreliable with TAU
TAU does not have sufficient reliability or validity, because: (a) the bladder often needs to be adequately filled at the time of the TAU to obtain an interpretable image, resulting in elongation of the cervix and masking of any funneling of the internal os; (b) fetal parts can obscure the cervix, especially >20 weeks gestation; (c) the relatively long distance from the probe to the cervix results in lower image quality compared to TVU; (d) patient obesity can result in poor imaging; and (e) excessive manual pressure may falsely elongate and distort CL. These factors undermine the reliability and validity of results derived by TAU.
2. Most women undergoing TAU CL screening need a TVU CL screen anyway
To ensure high sensitivity for detection of a short CL, usually defined as a TVU CL <25 mm, the cutoff used for TAU CL needs to be somewhat high, usually ≥35 mm ( Table ). In the largest study to date, a blinded study, 60% of women had a TAU CL cutoff necessitating a TVU. If one considers also 6% of women screened with TAU could not have the CL visualized by TAU, about two thirds of women screened by TAU end up needing TVU anyway. Moreover, 4-7% of short TVU CLs <25 mm would not be detected at all by TAU CL screening. A false-negative measurement raises clinical and legal concerns for the patient and the practitioner. There are no data comparing patient preferences between TAU CL screening and TVU CL screening. TVU is very well accepted, as 99% of women would have it again, and <2% experience severe pain.
| Study | Gestational age, wk (mean) | No. of women (with TVU CL <25 mm) | Bladder status at TAU screening | TAU and TVU results blind to each other | TAU cutoff, mm | TAU longer/shorter than TVU | TAU CL not attainable | Sensitivity a | Follow-up TVU needed |
|---|---|---|---|---|---|---|---|---|---|
| To et al, 2000 | 22–24 (23) | 149 | Prevoid (bladder volume calculated) | NR | NR | NR | 51% | NR | NR |
| Saul et al, 2008 | 14–34 (22) | 191 (14) | Postvoid | Yes | ≤30 | Similar | NR | 100% | NR |
| Stone et al, 2010 | 18–20 | 203 | Postvoid | No | NR | Shorter | NR | NR | NR |
| Hernandez-Andrade et al, 2012 | 6–39 (24) | 220 (20) | Prevoid | Yes | ≤25 ≤30 | Longer Longer | NR | 43% 57% | NR |
| Friedman et al, 2013 | 18–24 (20.5) | 1217 (76) | Prevoid Postvoid | No No | ≤36 <35 | Shorter Shorter | 6% 17% | 96% 96% | 60% NR |
| Rhoades et al, 2016 | 17–23 (20) | 404 | Postvoid | No | ≤35 | Shorter | 21% | 93% | 32.4% |
| Marren et al, 2014 | 18–20 (19) | 198 (13) | Prevoid Postvoid | No No | <30 <30 | Longer Similar | 3% 18% | “Poor” b 39% | NR |
3. Proven treatment for a short cervix was assessed by TVU, not TAU
All randomized controlled trials that have assessed treatment interventions (eg, vaginal progesterone, cerclage, pessary) for SPTB prevention based on short CL have used TVU. No randomized controlled trial on an intervention for short CL has used TAU for screening. A recent real-world study reported a significant decrease in preterm rates with adoption of TVU CL screening. No such real-word effectiveness has ever been published, at least so far, for TAU CL screening.
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