There is research being done into these positive/negative chromosomal tests, and specifically how some cells at the 8-cell stage become the baby, and other cells become the placenta and membranes, and how some "mosaics" might indeed have had all the abnormal cells go to the placenta and the normal cells go to the baby, and there's no way to tell with a current amnio or choronic villus sampling.
p.s. I think that baby girl in the first spoiler in post #5141 is Shiloh as a toddler. I saw those programs, too; she was quite intelligent, but her facial swelling (and ultimately her death) were due to antirejection drugs because she'd had at least two kidney transplants. She had a cloaca formed (combined uro- and colostomy) and got one kidney from a parent, and another from a stranger. She did have at least one kidney but it didn't function.
Here is a link to the Wiki page about sirenomelia, which I will spoiler because it does include graphic photos. In addition to mentioning Shiloh, they also discuss a Peruvian girl who lived into her teens, and a Florida woman, Tiffany Yorks who died a few years ago at age 27. I couldn't find a cause of death for Ms. Yorks, but I heard at the time that it was unrelated to her disability.
One of my Facebook friends, a former co-worker who lost a baby mid-trimester a couple years ago, recently gave birth to a daughter with caudal regression syndrome, which is precipitated by uncontrolled maternal diabetes during the first trimester. She is still in the NICU, and was originally thought to have sirenomelia but later scans indicated that she has two legs, which are curved and don't seem to have knees.
