I just had a transvaginal ultrasound that said that there was a "5mm hyperechoic endometrial mass". Is there any chance this is a decidual reaction?

Sonohystogram. If an endometrial mass was seen on transvaginal sonogram this is typically followed up with another test that allows visualization of the area. A specialized sonogram called a sonohystogram is a simple next step. Some doctors can look at the area in question with a camera called a hysteroscope. Endometrial polyps or submucosal fibroids are the most likely cause.