Two screens to predict spontaneous preterm birth in low-risk women, which were rapidly adapted into clinical practice despite a lack of supportive evidence, proved to have little predictive value in a large cohort study.

Transvaginal ultrasound examination for short cervical length and quantitative cervicovaginal swabbing for fetal fibronectin are routinely used to predict spontaneous preterm birth. To assess the accuracy of both screens individually and in combination, researchers analyzed data for women participating in the prospective multicenter Nulliparous Pregnancy Outcomes Study: Monitoring Mothers-to-Be ( nuMoM2b ). They focused on 9,410 nulliparous women with singleton pregnancies who were followed at eight clinical centers across the country. In addition to regular pregnancy visits, these women underwent both screening procedures at approximately 12 weeks, 19 weeks, and 28 weeks.

A total of 474 women (5%) had spontaneous preterm births, while 8,601 had term births and 335 had medically-indicated preterm births, reported M. Sean Esplin, MD, of Intermountain Healthcare and the University of Utah Health Sciences Center, Salt Lake City, and his associates.

Both screens had relatively low sensitivity and low positive predictive value, regardless of when they were performed, which threshold values were used, and whether the results were considered individually or in combination ( JAMA. 2017;317[10]:1047-56 ).

Transvaginal cervical length at 22-30 weeks’ gestation was the most accurate predictor of spontaneous preterm birth before 37 weeks, outperforming fetal fibronectin assessment alone. However, with a threshold of 25 mm or less – the most commonly used clinical cutoff – cervical length screening identified just 23.3% of spontaneous preterm births before 37 weeks. Use of that same threshold at 16-22 weeks – the most common time for screening in clinical practice – identified just 8% of subsequent spontaneous preterm births. The addition of fetal fibronectin did not increase the predictive performance of cervical length alone, according to the findings.

The researchers cited the low incidence of short cervix (1% at 16-22 weeks) as a potential reason why the ultrasound screen was not useful. “Using the most conservative threshold of 25 mm or less in the most common time for clinical screening (16-22 weeks’ gestation), 247 women would need to be screened to identify 1 case of spontaneous preterm birth,” the researchers wrote. Using a transvaginal cervical length of 15 mm or less during the same time period, the number needed to screen rose to 680 to identify a single case of spontaneous preterm birth.

These findings do not support the routine use of these two screens among nulliparous women with singleton pregnancies, the researchers wrote. Screening procedures with relatively poor predictive values “may sometimes be useful if they are inexpensive, lack serious adverse effects, and address a serious condition for which an effective intervention exists. Neither of these tests, alone or in combination, meets all of these criteria,” they added.

The Eunice Kennedy Shriver National Institute of Child Health and Human Development supported the study. Dr. Esplin reported holding a patent for serum markers of preterm birth and ties to Sera Prognostics and Clinical Innovations. One of his coauthors reported ties to Natera, Sequenom, Illumina, March of Dimes, Ariosa Diagnostics/Roche, KellBenx, and LabCorp.