This was the "right" fetus of a pair of third trimester fetuses. Growth had been normal and this examination was simply to assess "growth". These fetuses had had 4 previous scans and no unusual findings had ever been noted.The following image was obtained: (no video was available to produce a clip).

Well the obvious "finding" is the hypoechoic lesion behind the spine at the level of the heart.  We were quite baffled by the finding since it suggested a small myelocele or some sort of cyst. Yet, we had no other evidence of a neural tube defect and we had several previous exams done by several observers, including "double scan" in the second trimester (we have 2 observers scan each 2nd trimester baby). All cranial findings were normal (no fruit signs in particular). The exact level of the lesion was not recorded in previous exams (this is the level of the heart and previous views of the heart did not demonstrated the spine well due to overlying shadows.

We were quite stunned that we could have missed such a myelocele, so we ask to repeat th examination at no charge. Unfortunately the patient delivered before we could get to her. Both twins were totally normal at delivery. We rescanned both twin at 1 week of age and these are the relevant images at the level of the "lesion":

In conclusion the "lesion" we observed was an Artifacts . Ultrasound is an unusual discipline in which the first question regarding a lesion is always (or should always) be: is it real or is it an artifact. In this case I have no explanation on how this appeared.