Consistency in the transabdominal ultrasound measurement of cervical length in mid-pregnancy.

Purpose: The Western Australian Preterm Birth Prevention Initiative recommends a transabdominal cervical length (TACL) measurement at the mid-pregnancy ultrasound to screen low-risk women for preterm birth risk. In view of this recommendation, we assessed the inter-observer consistency of TACL screening in mid-pregnancy.

Methods: Routinely collected mid-pregnancy TACL ultrasound images were graded from 0 to 4 according to the anatomical landmarks identified by a single expert. A random selection of 10 images of each grade were disseminated in an electronic survey to determine inter- and intra-observer variations in the classification of the cervical image.

Results: A total of 244 participants graded 50 TACL images. Six participants repeated the grading. Overall agreement to the exact initial grade for all images was 49.6%, highest for images at both ends of the spectrum (83% Grade 0 and 70.4% for Grade 4). Overall agreement to the initial diagnostic Grades 3 and 4 was 75.3% (95% CI 74.5-76.0%) and was higher when the maternal bladder was empty. There was moderate inter-rater agreement (κ = 0.42) for Grades 3 and 4 (diagnostic) or Grades 1 and 2 (non-diagnostic). The intra-rater agreement was fair to good (κ = 0.59, 95% CI 0.49-0.70) for those who repeated the assessment (including the expert grader).

Conclusions: Sonographic CL screening is considered an important tool for the identification of women at high risk of preterm birth. Image classification of TACL performed poorly compared with previous studies assessing transvaginal cervical length. Improved reliability and measurement consistency may be achieved through high levels of quality assurance, ongoing training and image audit.