By John C. Hobbins, MD
Professor, Department of Obstetrics and Gynecology, University of Colorado School of Medicine, Aurora
Dr. Hobbins reports no financial relationships relevant to this field of study.
A large 10-center study has validated the concept of screening all patients who are presenting with preterm contractions in the third trimester with cervical length examinations by transvaginal sonography and using fetal fibronectin selectively only in those with cervical lengths between 1.5 and 3.0 cm.
Van Baaren GJ, et al. Predictive value of cervical length measurement and fibronectin testing in threatened preterm labor. Obstet Gynecol 2014;123:1185-1192.
The rate of preterm birth (PTB) in the United States has dropped slightly over the last few years, but it is still unacceptably high at 11.5%.1 Early delivery puts significant stress not only on the affected patients and their babies but also on our obstetrical health care system. Cervical length (CL) measurements by transvaginal sonography (TVS) and fetal fibronectin (fFN) testing had been introduced as tools to predict which pregnancies are most at risk for preterm birth. However, in the last few years, studies have surfaced evaluating these tools, specifically in the third trimester, as a way to determine which patients with preterm contractions (PTC) are truly in preterm labor. Until now, most of the trials have only looked at each method independently and have had small numbers of subjects.
A group from the Netherlands recently evaluated data accumulated from 10 hospitals between 2009 and 2012.2 Seven hundred eight women with documented preterm contractions between 24 and 34 weeks of gestation were managed according to a single protocol, which called for fFN specimens to be initially collected from the posterior fornix followed by CL assessments by TVS. The outcome variable was delivery within 7 days of presentation.
After exclusions were applied, 702 remained in the study, of which 80 women (12%) delivered within 7 days. The median gestational age at time of entry was 29 weeks. Interestingly, in those who delivered within this window, the average time of delivery was 2.2 days after entry. Both CL and fFN were reasonably predictive of PTB < 7 days, but the two tests in combination performed better. Not surprisingly, 47% of patients with CLs < 1.5 cm delivered < 7 days. Conversely, less than 1% of the patients with CLs > 3 cm delivered < 7 days. Based on these results, the authors applied a "contingency" approach to the data that would involve performing a CL on everyone first and then using the fFN only in those whose CLs were in an intermediate category of 1.5-3.0 cm. With this sequential method, the chances of the 149 fFN negative patients in this intermediate category of delivering within 7 days was only 2.1%. The remaining 148 fFN positive patients in this category had a 14% chance of delivery within 1 week.
This protocol would eliminate the need to admit and treat more than half of the patients (404 of the original 708) in the study, since their total risk of immediate delivery was 1.4%. The remaining 40% (which included everyone with CL < 1.5 cm and those with CL 1.5-3.0 cm who had positive fFN) would require justifiable in-hospital attention, since that group would have about a 25% risk of delivering sometime during the next 7 days.
COMMENTARY
When walking through any antepartum service in a tertiary care hospital, one will see that the majority of the patients housed there have been labeled with the diagnosis of "preterm labor" or "arrested preterm labor." As suggested by the above study and others, the vast majority of patients with PTC are not in true labor and, therefore, not in need of the unnecessary, and in most cases, unasked for attention that they get. Also, with this simple protocol, the expenditure of millions of dollars’ worth of hospitalizations can be averted.
The success of this simple protocol might have been anticipated from the results of an earlier study by Gomez et al.3 The thrust of that study was to show that the two methods (CL and fFN), used together, were more predictive of true preterm labor than CL alone. Actually, tucked within their data was similar evidence displaying the worth of using fFN selectively. Specifically, if the CL was > 3.0 cm, fFN added little to the negative predictive value. Other trials of this kind have suffered from small numbers. This study not only had adequate numbers of patients, but also put into play a rigid protocol employed by the 10 centers, thus enabling the authors to demonstrate the usefulness of CL in all patients with preterm contractions followed by a selective use of fetal fibronectin. By inserting the swab into the posterior fornix before doing the TVS, potential contamination of the fFN specimen can be avoided. If the CL is between 1.5 and 3.0, the specimens then can be sent off for testing. The rest can be discarded.
References
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Hamilton BE, et al. Births: Preliminary data for 2012. Natl Vital Stat Rep 2013;63:1-20.
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Van Baaren GJ, et al. Predictive value of cervical length measurement and fibronectin testing in threatened preterm labor. Obstet Gynecol 2014;123:1185-1192.
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Gomez R, et al. Cervicovaginal fibronectin improves the prediction of preterm delivery based on sonographic cervical length in patients with preterm contractions and intact membranes. Am J Obstet Gynecol 2005;92:350-359.
