Transvaginal ultrasound is the first-line diagnostic test for abnormal uterine bleeding (AUB) because it provides detailed visualization of the endometrial thickness, uterine contour, and adnexal structures. It helps to identify structural causes such as polyps, fibroids, hyperplasia, or malignancy. This modality is non-invasive, widely accessible, and cost-effective, allowing prompt assessment and guiding further management. Unlike other imaging techniques, it offers superior resolution for pelvic organs, essential for accurate evaluation of AUB.

Reference: Berek & Novak's Gynecology, Jonathan S. Berek, 16th Edition.