Endometrial sampling is required if patients have Risk factors for endometrial cancer (eg, current or recent tamoxifen therapy) Persistent or recurrent bleeding Endometrial thickness > 4 to 5 mm determined during ultrasonography (as a follow-up test) In premenopausal women, endometrial thickness measurement is not used for evaluation for endometrial neoplasia because it varies across the menstrual cycle (1): During menstruation: 2 to 4 mm Early proliferative phase (cycle days 6 to 14): 5 to 7 mm Late proliferative phase: ≤ 11 mm Secretory phase: 7 to 16 mm MRI provides detailed images that are useful in planning surgery but is expensive and is not the first-line imaging test for patients with AUB.