Because of my age and a slightly high risk of down's on the screening tests, I had an amnio. The amnio came back negative for all the things we were worried about, but positive for Mosaic Isochromosome 20q. Which means that some of the cells evaluated during the amnio were normal but some had abnormalities in chromosome 20. There are only 25 reported cases and only about 4-5 research papers discussing the abnormality, and absolutely no conclusive evidence about what it really means. Of the 25 reported cases, 21 resulted in babies with no complications at all (although the longest study only studied the kids up to age 3 1/2, so there is very little long-term data). But the 4 other cases resulted in the babies having complications that weren't compatible with life. Researchers suspect that in the 21 cases, the abnormal cells weren't even in the baby, but instead were "other embryonic" cells (cells from the placenta instead of the fetus, for example) that get picked up during an amnio. And in the 4 cases with very bad outcomes, there was no consistency amongst the problems. Each of the 4 babies had some different but very serious complication, which means that they can't even say that this particular abnormality caused any of those babies' problems. It might have just been a coincidence and those babies would have had those complications even without the abnormal chromosome.
So I have all this really scary information about something that might not mean anything at all and I'm not sure what to do with it. The geneticist did an ultrasound at 18 weeks and everything looked completely fine and we're doing another ultrasound at 22 weeks to check again. Of the 4 cases with bad outcomes, those moms actually got amnios because the problem showed up first on a routine ultrasound. So the fact that our ultrasounds look good is definitely reassuring. So far, everything we know points towards everything being fine, but it's not easy to put the worry out of my mind.
I guess I wonder if anyone else knows anything about Mosaic Isochromosome 20q? But I suspect not since it's so rare. Maybe I just needed to write this down and send it out. It's hard to have no control over something so important.