Reproducibility of assessment of full dilatation Cesarean section scar in women undergoing second‐trimester screening for preterm birth

To develop a standardised method and assess the reproducibility of measuring caesarean section (CS) scar and CS niche and their position relative to the internal os of the uterine cervix by transvaginal ultrasound in pregnant women with previous full dilatation CS.This is a prospective single centre reproducibility study on women with singleton pregnancies and a previous full dilatation CS who underwent transvaginal ultrasound assessment of cervical length and CS scar characteristics at 14 to 24 weeks' gestation. CS scar was identified as a hypoechoic linear discontinuity in the myometrium at the anterior wall of the lower uterine segment or cervix, and a 'niche' identified as an indentation at the site of the scar with a depth of at least 2mm. CS scar distance was measured from the internal cervical os. CS scar niche was assessed in the sagittal and transverse planes (length, depth, width, residual and adjacent myometrial thickness). Qualitative reproducibility was assessed by agreement on visualisation of the scar and niche. Quantitative reproducibility of CS scar measurements was assessed in three groups of images: 1) real-time acquisition and caliper placement on two-dimensional (2D) images by two operators: 'Real-time 2D images'; 2) offline caliper placement by two operators on stored 2D images acquired by first operator: 'Offline 2D still images'; 3) three-dimensional (3D) volume manipulation and caliper placement on 2D images extracted by two operators: '3D volume images'. Agreement on CS scar visibility and presence of niche was analysed by kappa coefficient. Intra- and interobserver reproducibility of quantitative measurements were assessed using Bland-Altman plots.To meet the power calculation 72 women were recruited. The CS scar was visualised in ≥ 80% of images. Interobserver agreement for scar visibility and presence of niche in real-time 2D images was high (kappa coefficient 0.84 and 0.85, respectively). Reproducibility overall was higher for real-time 2D or offline 2D still images compared to 3D volume images. 95% limits of agreement for intraobserver reproducibility were within 1.1mm and 3.6mm; 95% limits of agreement for interobserver reproducibility were within 2.0mm and 6.3mm. CS scar distance to internal cervical os was the most reproducible 2D measurement (95% limits of agreement between 1.1 and 2.7mm). Niche measurements were the least reproducible measurements (95% limits of agreement between 1.6mm and 6.3mm). There was no consistent difference between measurements obtained by reacquisition of 2D images, caliper placement on 2D stored images or volume manipulation.The CS scar position and niche after a full dilatation CS can be assessed in the second trimester of the subsequent pregnancy using either 2D or 3D volume ultrasound imaging with high level of reproducibility. Overall the most reproducible CS scar measurement was the CS scar distance to internal cervical os. This proposed method will enable clinicians to reliably assess the CS scar, which may have potential to predict pregnancy outcome. This article is protected by copyright. All rights reserved.