(146) A novel first-trimester ultrasound marker for cesarean scar pregnancy: the tadpole sign

Objective: We aim to describe a first trimester ultrasound marker for cesarean scar pregnancy (CSP) diagnosis and compare it to the currently used “crossover sign".

Study Design: This is a retrospective case-control study of first trimester transvaginal ultrasound (TVUS) images from obstetric imaging databases. All images were in the midsagittal plane of the uterus with visualization of cervix to fundus. Cases were those with suspected CSP diagnosed radiographically. Controls were the next available first trimester TVUS of a viable singleton intrauterine pregnancy with prior cesarean delivery matched for gestational age. Images were independently reviewed by two experts. The distance from point "M" (as defined by the crossover sign) to the fundus was divided into thirds, with point A closest to fundus, B in the middle, and C at the “M-line”. At each point the anterior myometrial thickness was measured and compared between cases and controls. The visual “tadpole sign” is the presence of the gestational sac (the “head”) fixed anteriorly towards the cesarean scar, with closed uterine cavity extending cephalad towards the fundus (the “tail”). Analyses included interclass correlation coefficient (ICC) and area under the curve (AUC) with 95% confidence intervals (CI).

Results: We identified 52 CSP cases and 52 controls. Distribution of the myometrial thickness measurements at each of the three points are presented in image 1. The ICC for the measurements at points A, B and C were 0.88, 0.95, 0.86, respectively. The AUC and 95% CI for each measurement was 0.96 (95% CI 0.93-0.99), 0.98 (95% CI 0.94-1.0), 0.75 (95% CI 0.65-0.85), respectively. There was 100% agreement between the crossover sign and tadpole sign in identifying CSPs. Both the tadpole sign and crossover sign identified all CSPs accurately except one case.

Conclusion: The tadpole sign highly correlates with the crossover sign for diagnosis of CSP. Myometrial thickness correlates well to the presence of the tadpole sign. This novel sign is easily identified and carries strong interrater reliability without need for additional ultrasound image manipulation.