1065: Are rates of spontaneous preterm birth stable across a range of normal cervical lengths?

Abstract

Cervical length (CL) screening is an effective adjunct to reduce recurrent spontaneous preterm birth (sPTB) in high-risk women. While the efficacy of universal screening in women with no prior sPTB is debated, studies have shown benefit to identifying low-risk women with shortened CL ≤20 mm. In high-risk women a range of normal CLs (≥25 mm) has been associated with a stable risk of recurrent sPTB; however, it is unclear how rates of sPTB vary over a range of measured-normal CLs in low-risk women. Retrospective cohort of women with singletons and no prior sPTB who had universal mid-trimester transvaginal CL screening 1/2016-8/2018 and a measured (Normal) CL ≥25mm. We excluded fetal anomalies or an unmeasurable CL due to a poorly-developed lower uterine segment. The primary outcome was sPTB < 37 wk. We considered CL in interval groups: 25-29 mm, 30-34 mm, 35-39 mm, 30-44 mm, and ≥45 mm (χ2 test of trend) and as a continuum (logistic regression and ROC curve). A secondary outcome was the number of hospital evaluations for suspected preterm labor (PTL). 990 low-risk women were included. The incidence of sPTB < 37 wk was 3.6%; mean CL was 39.2±6.0 mm (interdecile range=31.6, 47.0), measured at 19.4±1.1 wk. The median number of PTL evaluations was 0 (range 0-4) and mean birth gestational age was 38±2.4 wk. There was a significant decreasing trend in rates of sPTB across increasing CL groups (p=0.001; Table). Increasing CL as a continuous variable was also associated with a significantly lower odds of sPTB (OR=0.92, 95% CI 0.87, 0.98). However, the number of evaluations for suspected PTL was not associated with CL (p=0.55). The ROC (Figure) AUC for sPTB was only 0.64, suggesting that a range of normal CLs in low-risk women was a poor predictor of sPTB. Among low-risk women with a normal CL observed during mid-trimester screening, there was only a modest effect of a range of measured-normal CLs on sPTB < 37 wk and no significant association with the number of hospital evaluations for PTL.View Large Image Figure ViewerDownload Hi-res image Download (PPT)