Mirror artifacts in obstetric ultrasound: case presentation of a ghost twin during the second-trimester ultrasound scan.

Mirror artifacts are produced by the reflection of ultrasound waves after they propagate through a structure and encounter a strong and smooth interface capable of acting as a mirror. Ultrasound waves bounce back and forth between the mirroring interface and the reflective object and then eventually return to the transducer. The typical display of the mirror artifact consists of two similar structures separated and at similar distances from the reflective interface. We report a mirror artifact in a patient with a singleton gestation at 18 weeks. The image was interpreted as consistent with a twin gestation using transabdominal and transvaginal ultrasound. The differential diagnosis consisted of an abdominal heterotopic pregnancy. The presence of synchronized but opposite movements of both fetuses, and the blurred image of the second fetus, suggested a mirror artifact. The reflective surface was created by the interface located between a distended rectosigmoid filled with gas and the posterior uterine wall. Mirror artifacts can lead to diagnostic errors. This case illustrates how a distended rectosigmoid colon can generate an image that simulates either a twin gestation or an abdominal heterotopic pregnancy.