Indexed on: 14 Sep '20Published on: 13 Sep '20Published in: American Journal of Obstetrics & Gynecology
A sonographic short cervix length (<25 mm during mid-gestation) is the most powerful predictor of preterm birth. Current clinical practice assumes that the same cervical length cut-off value should apply to all women when screening for spontaneous preterm birth, yet this approach may be suboptimal OBJECTIVES: 1) To create a customized cervical length standard that considers relevant maternal characteristics and gestational age at sonographic examination and 2) to assess whether customization of cervical length evaluation improves the prediction of spontaneous preterm birth. This retrospective analysis comprises a cohort of 7,826 pregnancies enrolled in a longitudinal protocol between January 2006 and April 2017 at the Detroit Medical Center. Study participants met the following inclusion criteria: singleton pregnancy, one or more transvaginal sonographic measurements of the cervix, delivery after 20 weeks of gestation, and available relevant demographics and obstetrical history information. Data from women without a history of preterm delivery or cervical surgery who delivered at term without progesterone treatment (N=5,188) were used to create a customized standard of cervical length. Prediction of the primary outcome, spontaneous preterm birth <37 weeks, was assessed in a subset of pregnancies (N=7,336) that excluded cases with induced labor prior to 37 weeks. Area under the receiver operating characteristic curve and sensitivity at a fixed false-positive rate were calculated for screening at 20-23, 24-27, 28-31, and 32-35 weeks of gestation in asymptomatic patients. Survival analysis was used to determine which method is better at predicting imminent delivery among symptomatic women. The median cervical length remained fundamentally unchanged until 20 weeks of gestation and subsequently decreased non-linearly with advancing gestational age among women who delivered at term. The effects of parity and maternal weight and height on the cervical length were dependent on the gestational age at ultrasound scan (interaction, p<0.05 for all). Parous women had a longer cervix than nulliparas, and the difference increased with advancing gestation after adjusting for maternal weight and height. Similarly, maternal weight was non-linearly associated with a longer cervix, and the effect was greater later in gestation. The sensitivity at 10% false-positive rate for prediction of spontaneous preterm birth <37 weeks by a short cervix ranged from 29% to 40% throughout pregnancy, yet it increased to 50%, 50%, 53%, and 54% at 20-23, 24-27, 28-31, and 32-35 weeks, respectively, for a low customized percentile (McNemar test: p<.001 for all). When cervical length <25 mm was compared to the customized screening at 20-23 weeks of gestation by using a customized percentile cut-off value that ensured the same negative likelihood ratio for both screening methods, the customized approach had a significantly higher (about double) positive likelihood ratio in predicting spontaneous preterm delivery at <33, <34, <35, <36, and <37 weeks of gestation. Among symptomatic women, the difference in survival between those with a customized cervical length percentile ≥10 and those <10 was greater than the difference in survival between women with cervical length ≥ 25 mm and those with cervical length <25 mm. Compared to the use of cervical length <25mm, a customized cervical length assessment i) identifies more women at risk for spontaneous preterm delivery; ii) improves the distinction between patients at risk for impending preterm delivery in those who have an episode of preterm labor. Copyright © 2020. Published by Elsevier Inc.