The taper sign: Sonographic representation of uterine dehiscence in severe cases of placenta accreta spectrum

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Abstract
Introduction: Prenatal diagnosis of placenta accreta spectrum (PAS) depends on reliable ultrasound markers and skilled image interpretation. Existing PAS markers emphasize placental features suggestive of abnormal adherence and require advanced image optimization. However, emerging hypotheses suggest that uterine dehiscence—not placental invasion—is the central mechanism underlying PAS, particularly in severe (Grade 3) disease. Despite this, few established ultrasound markers address this uterine dehiscence. We describe a novel ultrasound marker, the taper sign, which uniquely represents the uterine dehiscence in severe PAS with anterior placenta previa, and outline a standardized technique for image acquisition and interpretation. Methods: This descriptive report details the sonographic appearance of the taper sign on grayscale ultrasound in pregnancies with anterior placenta previa and prior cesarean delivery with surgically and histopathologically confirmed Grade 3 PAS. Ultrasound images from cases at our center were reviewed to characterize the morphologic features of the taper sign and define optimal imaging parameters. Representative images illustrate this novel marker and the required anatomical landmarks. Results: The taper sign is characterized by progressive myometrial thinning until it disappears at the level of the prior cesarean scar, producing a wedge‐shaped appearance; this is distinct from previously described PAS markers. It is best seen using transvaginal ultrasound with a partially full bladder. Beginning in a midsagittal 2D grayscale view of the cervix, the anterior cervical lip is traced proximally as it transitions into the lower uterine segment myometrium. With the placenta previa bulging anteriorly, the overlying myometrium tapers to a point, terminating at the margin of the uterine dehiscence. This correlates with the deepest placental infiltration seen in Grade 3 PAS. In our experience, the taper sign is easily obtained and consistently identified in Grade 3 PAS with anterior placenta previa and prior cesarean delivery. Conclusion: The taper sign represents a novel ultrasound marker of severe PAS, reflecting the distinct wedge‐like appearance of attenuated myometrium as it terminates at the uterine dehisence. This report provides a standardized description and imaging protocol to facilitate recognition of patients at increased risk for severe maternal morbidity. Ongoing studies aim to validate this marker's diagnostic performance.

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