38-year-old mother scanned at 21 weeks.

And you also get a neat video clip (only 190 kB).

I had also mentioned (to save on my infamous "huh huh" answers) that this was not a little boy (visible girl on the clip) and that this was not an ovarian cyst (too easy...)

And the question was: " What does this little girls has and what is the prognosis ?"

Well it proved to be a difficult case since no one got the answer !

Observations

There were several important observations:

1) it is a little girl

2) the first image demonstrated some mild pyelectasis

3) the amniotic fluid was normal

4) there is a "cystic collection"

All these observations are pretty straight-forward, the only that requires some further delineation was the "cystic collectionâ€:

1. That "cystic collection†is reasonably clear (the echoes inside appear more due to side-lobe artifacts than real "debrisâ€.
2. It is strictly midline in the pelvis but deviates in the abdomen
3. And mostly, image 4 showed that the intra-abdominal portion of the umbilical arteries surrounded it.

That third criterion is the most important observation, and it tells us that this is a bladder. No other structure can really present surrounded by the intra-abdominal portion of the umbilical arteries. Ovarian cysts cannot do that (those of you who do pelvic ultrasound in women know that very well) and neither can a hydrometrocolpos present this finding.

Putting it together

The question then is: What condition can present in a little girl, with a distended bladder but normal amniotic fluid. Interestingly enough, it is a distended bladder, not an obstructed bladder since there is only mild pyelectasis and clearly the amniotic fluid is not decreased. This, therefore, eliminates urethral agenesis. The lack of finding inside the bladder also makes a prolapsed ureterocele in the neck of the bladder unlikely (these look like a little cyst in the neck of the bladder).

The most likely diagnosis then is the megacystis-microcolon-hypoperistalsis syndrome, nowadays called the Intestinal hypoperistalsis, megacystis-microcolon syndrome. Click on the link for more info on this syndrome.