Timing of mid‐trimester cervical length shortening in high‐risk women

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Abstract
Objective To examine the natural history of cervical length shortening in women who had experienced at least one prior spontaneous preterm birth at between 17 + 0 and 33 + 6 weeks' gestation. Methods This was an analysis of prerandomization data from the multicenter Vaginal Ultrasound Cerclage Trial. Serial cervical length was measured by transvaginal sonography in 1014 high‐risk women at 16 + 0 to 22 + 6 weeks. We performed survival analyses in which the outcome was cervical length shortening < 25 mm and data were censored if this did not occur before 22 + 6 weeks' gestation. The incidence of cervical length shortening and the time to shortening were compared for women whose earliest prior preterm birth was in the mid‐trimester, defined as < 24 weeks, vs. those at weeks 24–33. Similar comparisons were performed based on each patient's most recent birth history. Results Time to cervical length shortening by survival analysis was significantly shorter (hazard ratio (HR) = 2.2, P < 0.0001) and the relative risk (RR) of shortening significantly higher (RR = 1.8, P < 0.0001) for women whose earliest prior spontaneous preterm birth was at < 24 weeks. A larger effect was observed for women whose most recent birth was at < 24 weeks (HR = 2.8, P < 0.0001; RR = 2.1, P < 0.0001). The observed hazard ratios remained significant after adjusting for confounders in a multivariable Cox proportional hazards model. Conclusion Women with a prior spontaneous preterm birth at < 24 weeks are at a higher risk of cervical shortening, and do so at a higher rate and at an earlier gestational age, than do women with a later preterm birth history. Copyright © 2008 ISUOG. Published by John Wiley & Sons, Ltd.