What Is Threatened Preterm Labor? Definition and Diagnostic Criteria
Clinically, threatened preterm labor (TPTL) is the presence of symptoms suggestive of labor, ranging from mild back pain or pelvic pressure to palpable, painful contractions, occurring between 20 and 37 weeks of pregnancy.
Gestational limits vary slightly by source. The American College of Obstetricians and Gynecologists (ACOG) defines preterm birth in general terms as delivery between 20+0/7 and 36+6/7 weeks of gestation, while The World Association of Perinatal Medicine and the Perinatal Medicine Foundation (WAPM-PMF) guideline sets the upper limit for PTL at 37+0 weeks, with a lower gestational threshold that is individualized by setting, 22+0 weeks in high-income settings, and as late as 26+0 weeks in resource-limited settings, depending on local neonatal capacity (Dagklis et al., 2023).
Both frameworks agree on the scale of the underlying problem: fewer than 10% of women with a clinical diagnosis of preterm labor actually give birth within 7 days. Symptoms alone are poor predictors of outcome.
Threatened Preterm Labor vs. Established Preterm Labor
The WAPM-PMF guideline reserves the term “established PTL” for labor beyond which delivery is considered inevitable, defined by two necessary criteria: at least six contractions per 30 minutes and cervical dilation of 3 cm or more. Below that threshold, a patient remains, by definition, in the threatened category.
ACOG notes that regular preterm contractions are common but do not reliably predict which women will develop progressive cervical change. Contraction frequency alone cannot separate patients heading toward imminent delivery from the much larger group who will not deliver soon. Clinicians need a second, more objective axis: cervical status.
Initial Assessment: Cervical Examination and Biomarkers
The WAPM-PMF guideline recommends combining cervical length (CL) measurement with biochemical markers, where available, to stratify risk in symptomatic women. Transvaginal ultrasound is the gold-standard technique for CL. In a meta-analysis of symptomatic women, a CL under 15 mm predicted 60% of deliveries within 7 days, while a CL over 15 mm carried a negative predictive value of 96%, reassuring most patients that delivery is unlikely in the short term (Sotiriadis et al. 2009).
Biochemical markers, fetal fibronectin (fFN), PAMG-1, and phIGFBP-1, add value particularly in the intermediate CL range of 15–30 mm, where cervical length alone cannot resolve risk. However, ACOG cautions that the positive predictive value of a positive fFN test or a short cervix alone is poor and should not be used exclusively to direct management in acute symptomatic settings.
Identifying women with preterm labor who ultimately will give birth preterm is difficult. As demonstrated by a prospective cohort study, the large majority of spontaneous preterm births occurred in women with a normal cervix: of the women who delivered spontaneously before 37 weeks, 82% had a cervical length within the limits (25 mm or greater) (Orzechowski et al., 2014).
This is the diagnostic space where torsional wave elastography (TWE), a novel alternative to shear wave elastography, offers a complementary, objective measurement. Unlike ultrasound-based cervical length, which assesses geometry, TWE quantifies cervical tissue stiffness directly. It is an essential asset considering that cervical remodeling is a biochemical process and the softening of the cervical stroma is shown before any of that change becomes visible as shortening.
CerviSense, Innitius’s CE-marked intravaginal device, converts this measurement into a stiffness reading in kilopascals within seconds. Its reproducibility for threatened preterm labor diagnosis has been validated in a multicenter study, reporting interobserver ICC of 0.93 (Molina et al., 2023). This positions stiffness assessment as a candidate addition to the diagnostic toolkit already anchored around CL and biomarkers.
Bed Rest in Threatened Preterm Labor: What the Evidence Shows
Bed rest has a long history in TPTL management, but the evidence does not support it. ACOG states that reduced activity and hydration have not been shown to be effective for preventing preterm birth and should not be routinely recommended, citing risks including thromboembolic events and bone demineralization.
The WAPM-PMF guideline goes further, issuing a strong recommendation against activity restriction based on moderate-quality evidence. A secondary analysis found that activity restriction actually increased preterm birth rates in asymptomatic nulliparous women with a short cervix compared to normal activity (Grobman et al. 2013). A subsequent randomized trial in women with arrested preterm labor found no reduction in preterm birth with restricted activity (Saccone et al. 2023). Associated harms include venous thrombosis, muscle atrophy, and cardiovascular deconditioning, alongside psychological costs such as self-blame.
Hospital vs. Outpatient Management Pathways
Because roughly 30% of preterm labor resolves spontaneously and half of hospitalized patients ultimately deliver at term, indiscriminate admission is neither efficient nor necessary. The WAPM-PMF guideline proposes a stepwise approach: a minimum two-hour observation period, followed by a disposition decision based on available resources. In low-resource settings, discharge is guided by contraction cessation and absence of cervical change.
Where ultrasound and biomarkers are available, the cut-off is established as CL of 15 mm or greater alone, a negative biomarker alone, or CL above 30 mm (or 15–30 mm with a negative biomarker) when both are available. This contingent model, using CL as the primary filter and biomarkers to resolve intermediate-risk cases, allows hospitals to reserve admission, tocolysis, and corticosteroid administration for patients genuinely likely to deliver soon, while safely discharging the majority who are not.
Technology has evolved to offer a fast, bedside stiffness reading that fits naturally into this two-hour observation window. CerviSense delivers a cervical stiffness measurement in kilopascals within seconds of insertion, fast enough to inform the same disposition decision without extending the observation period itself. That reading feeds an AI-driven risk model built specifically to flag threatened preterm labor. This application is currently under clinical validation, and we are now running a clinical trial across 18 sites in Germany and Spain to assess whether the device can predict spontaneous preterm birth within 7 days.
Sources
American College of Obstetricians and Gynecologists’ Committee on Practice Bulletins—Obstetrics. Practice Bulletin No. 171: Management of Preterm Labor. Obstet Gynecol. 2016 Oct;128(4):e155-64. doi: 10.1097/AOG.0000000000001711.
Dagklis T et al. Management of preterm labor: Clinical practice guideline and recommendation by the WAPM-World Association of Perinatal Medicine and the PMF-Perinatal Medicine Foundation. Eur J Obstet Gynecol Reprod Biol. 2023 Dec;291:196-205. doi: 10.1016/j.ejogrb.2023.10.013.
Grobman WA et al. Activity restriction among women with a short cervix. Obstet Gynecol. 2013 Jun;121(6):1181-1186. doi: 10.1097/AOG.0b013e3182917529.
Molina FS et al. Reproducibility and usability assessment of the novel Fine Birth device for threatened preterm labor diagnosis. Am J Obstet Gynecol MFM. 2023 Jul;5(7):100982. doi: 10.1016/j.ajogmf.2023.100982.
Orzechowski KM et al.A universal transvaginal cervical length screening program for preterm birth prevention. Obstet Gynecol. 2014 Sep;124(3):520-525. doi: 10.1097/AOG.0000000000000428.
Saccone G et al. Activity restriction for women with arrested preterm labor: a randomized controlled trial. Am J Obstet Gynecol MFM. 2023 Aug;5(8):100954. doi: 10.1016/j.ajogmf.2023.100954.
Sotiriadis A et al. Transvaginal cervical length measurement for prediction of preterm birth in women with threatened preterm labor: a meta-analysis. Ultrasound Obstet Gynecol. 2010 Jan;35(1):54-64. doi: 10.1002/uog.7457.