Longitudinal changes in cervical length prior to the onset of spontaneous labor at term gestation.

BackgroundCommon pathways of parturition include myometrial contractility, choriodecidual activation, and cervical shortening/remodeling. Currently, cervical component assessment is usually performed by digital examination, but this method is invasive, subjective, and inaccurate in evaluating the internal os. Cervical sonography provides an objective assessment of cervical effacement and other changes.ObjectiveThe aims of this study were to: (1) characterize cervical length profile prior to the onset of spontaneous labor at term; (2) compare the cervical length profile between nulliparous and multiparous women; (3) evaluate the association between the magnitude of cervical length changes and the onset of spontaneous onset of labor within 24- or 48-hours after cervical length assessment.Study designA prospective longitudinal cohort study.SettingLabor and delivery unit, Bangkok, Thailand.MethodsA total of 135 patients were enrolled in this study. Thirty pregnant women met the exclusion criteria. Therefore, 105 patients who underwent transvaginal cervical length measurements every 3 to 4 days at 37 to 38 weeks and every 2 to 3 days thereafter until the spontaneous onset of labor were included in the analysis.Main outcome measuresThe primary outcome was the time interval from each cervical length assessment to the spontaneous onset of labor at term, defined as the interval between the date of the cervical length examination and the onset of spontaneous labor. Secondary outcomes included the association between the magnitude of cervical length changes and the subsequent spontaneous onset of labor within 24 or 48 hours after a cervical length assessment. Spontaneous onset of labor is defined as regular uterine contractions occurring at a frequency of at least 4 in 20 minutes for a minimum of 1 hour with either cervical dilatation ≥4 cm or effacement of ≥80% or cervical changes, or spontaneous rupture of membranes, followed by delivery within 24 hours of the onset of labor.Results(1) Cervical length demonstrated a gradually progressive reduction during the few weeks preceding the spontaneous onset of labor, with an evident reduction in 48 hours, (2) nulliparous and multiparous women had similar absolute cervical length profile; (3) among the cervical length changes patterns, absolute cervical length changes from the previous visit have the highest discrimination ability for the identification of spontaneous labor within 24 hours; (4) absolute cervical length reduction of >5 mm compared with the prior visit was associated with a significantly higher hazard of spontaneous onset of labor within 24 hours (hazard ratio 11.76; 95% CI: 3.27-42.28; P<.001), after adjusting for maternal age, BMI, parity, gestational age at measurement and baseline cervical length.ConclusionCervical length demonstrated a gradually progressive reduction during the few weeks preceding the spontaneous onset of labor, with an evident reduction in 48 hours. Absolute cervical length reduction of >5 mm compared with the prior visit was associated with a significantly higher hazard of having a spontaneous onset of labor within 24 hours (hazard ratio, 11.76). This knowledge may help identify women at an increased likelihood of spontaneous onset of labor within 24 hours.