Prediction of Spontaneous Preterm Birth in Women with Previous Full Dilatation Cesarean Delivery

Background Previous term (≥37 weeks’ gestation) full dilatation cesarean delivery (FDCD) is associated with an increased risk of subsequent spontaneous preterm birth (sPTB). The mechanism is unknown. We hypothesized that cesarean delivery (CD) scar characteristics and scar position relative to the internal cervical os may compromise cervical function leading to shortening cervical length (CL) and sPTB. Objective To determine the relationship between CD scar characteristics and position assessed by transvaginal ultrasound in pregnant women with previous FDCD and the risk of shortening CL and sPTB. Study design This was a single-center prospective cohort study of singleton pregnant women (14 to 24 weeks’ gestation) with previous term FDCD attending a high-risk preterm birth surveillance clinic (2017–2021). Women underwent transvaginal ultrasound assessment of CL, CD scar distance relative to the internal cervical os and scar niche parameters using a reproducible transvaginal ultrasound technique. sPTB prophylactic interventions (vaginal cervical cerclage or vaginal progesterone) were offered for short CL (≤25mm) or in women with a previous history of sPTB/late miscarriage after FDCD. Primary outcome was sPTB; secondary outcomes included short CL and need for prophylactic interventions. Multivariable logistic regression analysis was used to develop multiparameter models combining CD scar parameters, CL, previous FDCD history and maternal characteristics. Model predictive performance was examined using area under the receiver-operating-characteristics curve (AUC) and detection rate (DR) at various fixed false-positive rates (FPR). The optimal cut-off for CD scar distance to best predict short CL and sPTB was analyzed. Results CD scar was visualized in 90.5% (220/243) of women. sPTB rate was 4.1% (10/243) and 12.8% (31/243) of women developed a short CL. History (n=4) or ultrasound (n=19) indicated cervical cerclage was performed in 23/243 (9.5%) women; 2/23 (8.7%) delivered spontaneously preterm. Multiparameter model based on absolute scar distance from the internal os best predicted sPTB (AUC 0.73, 95%CI, 0.57-0.89, DR of 60% for a fixed 25% FPR). Models based on the relative ‘anatomical’ position of the CD scar to the internal os and CD scar position with niche parameters (length, depth and width) best predicted development of short CL (AUC 0.79, 95%CI 0.71-0.87 and 0.81, 95%CI 0.73-0.89 respectively; DR of 73% at a fixed 25% FPR). sPTB was significantly more likely when the CD scar was <5.0mm above or below the internal os, aOR 6.87 (95%CI 1.34-58; p=0.035). Conclusion In pregnancies following FDCD, CD scar characteristics and distance from the internal os predict women at risk of sPTB and developing short CL. Overall, the sPTB rate was low, but it was significantly increased in women with a scar located <5.0mm above or below the internal cervical os. Shortening of CL was strongly associated with a low scar position. Our novel findings indicate that a low CD scar can compromise the functional integrity of the internal cervical os leading to cervical shortening and/or sPTB. Assessment of CD scar characteristics and position appear to have utility for preterm birth clinical surveillance in women with previous FDCD and could better select women who would benefit from prophylactic interventions.