What's the difference between a pelvic ultrasound and a transvaginal ultrasound?

Nearly same...: There are two ways to image the pelvic organs. One is by transabdominal route (transducer in placed on the skin of lower abdomen with a distended urinary bladder); and the other is transvaginal ultrasound where the transducer is placed inside the vagina to assess pelvic organs. Transvaginal ultrasound means just that; whereas, pelvic ultrasound includes both transabdominal and transvaginal US.