The capacity of mid-pregnancy cervical length to predict preterm birth in low-risk women: a national cohort study.

INTRODUCTION We investigated the predictive capacity of mid-trimester cervical length (CL) measurement for spontaneous and iatrogenic preterm birth. MATERIAL AND METHODS We performed a prospective observational cohort study in nulliparous women and low-risk multiparous women with a singleton pregnancy between 16(+0) and 21(+6) weeks of gestation. We assessed the prognostic capacity of transvaginally measured mid-trimester CL for spontaneous and iatrogenic preterm birth (<37 weeks) using likelihood ratios (LR) and receiver-operating-characteristic analysis. We calculated numbers needed to screen to prevent one preterm birth assuming different treatment effects. Main outcome measures were preterm birth <32, <34 and <37 weeks. RESULTS We studied 11,943 women, of whom 666 (5.6%) delivered preterm: 464 (3.9%) spontaneous and 202 (1.7%) iatrogenic. Mean CL was 44.1 mm (SD 7.8 mm). In nulliparous women, the LRs for spontaneous preterm birth varied between 27 (95% CI 7.7-95) for a CL ≤ 20 mm, and 2.0 (95% CI 1.6-2.5) for a CL between 30 and 35 mm. For low-risk multiparous women, these LRs were 37 (95% CI 7.5-182) and 1.5 (95% CI 0.97-2.2), respectively. Using a cut-off for CL ≤ 30 mm, 28 (6.0%) of 464 women with spontaneous preterm birth were identified. The number needed to screen to prevent one case of preterm birth was 618 in nulliparous women and 1417 for low-risk multiparous women (40% treatment effect, cut-off 30 mm). CONCLUSION In women at low risk of preterm birth, CL predicts spontaneous preterm birth. However, its isolated use as a screening tool has limited value due to low sensitivity.