Cervical length and fetal fibronectin for the identification of true spontaneous onset of labor

Introduction: True labor is characterized by regular uterine contractions and cervical changes. However, the precise diagnosis of the “onset” of true labor is challenging, as the examination of cervical changes requires multiple timepoints, and it is a retrospective diagnosis. The objectives of this study are to determine the predictive performance of 1) cervical length, 2) cervicovaginal fluid fetal fibronectin, and 3) maternal angiogenic factors concentration [placental growth (PlGF), soluble fms-like tyrosine kinase-1 (sFlt-1), or its ratio (sFlt-1/PlGF)] for the identification of spontaneous true onset of labor at term within 24 h of the assessment in singleton pregnant women presenting with labor symptoms (i.e. differentiating true and false labor). Material and methods: Design: A prospective observational cohort study Setting: Labor and Delivery unit, Bangkok, Thailand Methods: 146 singleton pregnancies with symptoms of labor were enrolled. A total of 132 patients underwent transvaginal measurements of cervical length and angiogenic factors, with cervicovaginal fluid fetal fibronectin available in 90 cases. Main Outcome Measures: The primary outcome was the spontaneous onset of true labor at term, defined as regular uterine contractions occurring at a frequency of at least 4 in 20 min with either cervical dilatation ≥4 cm or effacement of ≥ 80% or cervical changes or spontaneous rupture of membranes, followed by delivery within 24 h of the onset of true labor in women at ≥37^th weeks of gestation. Predictive performance of individual biomarkers and the combined model was calculated to identify the spontaneous onset of true labor at term. Results: The combination of cervical length and fetal fibronectin yielded significantly better predictive performance than the individual markers, with a sensitivity of 62.1%, a specificity of 90.6%, a positive predictive value of 92.3%, a negative predictive value of 56.9%, as well as a positive and a negative likelihood ratio of 6.62 and 0.42, respectively, for the identification of spontaneous true onset labor at term with delivery within 24 h of the assessment. Conclusion: Cervical length measurement with fetal fibronectin had a high specificity, positive predictive value, and positive likelihood ratio, but low negative predictive value and moderate negative likelihood ratio for identifying the spontaneous true onset of labor at term. This test may be used as a bedside test to rule in the diagnosis of true labor. However, its moderate sensitivity and negative likelihood ratio, along with a low negative predictive value, indicate that a negative result cannot safely discharge patients and may limit its use as a standalone triage test, warranting the search for other biomarkers.