This patient was scanned at 14 weeks for abdominal pain and bleeding. A repeat scan the next day provided similar images. The video-clip is really important in this diagnosis and unfortunately to be representative it is fairly large (6 MB).

What is the finding or findings ?

The finding is that of a normal gestation in the uterus and ... another one that is less well defined. Many have suggested that this could be a: 

• heteropic pregnancy (the combination of an ectopic an eutopic pregnancy). Some cleverly explained the difference in size of the fetuses with superfetation 
• didelphys uterus (or other uterine anomalies)

This finding should send shivers in our spines ! This "second pregnancy" does not exist. It is a complete creation by the ultrasound machine. This is what is called an Mirror image artifact. 

This is how mirror image artifacts occur::

A pulse is emitted by the machine.

It hits a first interface in red on the drawing.

Part of the beam is reflected towards the transducer (represented as a yellow wave) and part of the beam is deflected according to the incidence angle as the oblique beam in blue.

The yellow segment returns to the transducer and forms the normal image. The blue segment continues until it hits another interface (in green). It bounces back on the green interface and returns to the red interface.

The blue segment then returns to the machine. The machine has no idea of what has happened and simply assumes that these echo come from deeper (they took longer to return) and the machine thus creates another image deeper in the patient.

What is truly remarkable in this case is that not only a decent-looking mirror image baby was created, but that even Doppler signal could be obtained in this "baby". 

In view of the suspicious clinical finding and despite having considered that this might be an artifact (even though it was visible on 2 consecutive days), the patient went to laparoscopy, and was noted on laparoscopy to be normal. 

I have had a similar case (did not go all the way to laparoscopy) and I had my case included in my lecture on Pitfalls and Artifact (where those frames come from). When Connie contacted me I was flabbergasted. I could not believe some one else had seen such an artifact. I included this in the lecture and for about 2-3 years now almost every time I give this lecture in a large audience I have someone come after the lecture to tell me that, they too have seen it. Two-three other patients have indeed also gone to surgery for that artifact. Among those who correctly answer this case, several had had similar situations.

I think this case is truly remarkable because we take so much for granted that what we see is what is there, that indeed very few who responded though this could be a mirror-image artifact.