Blood tests on an expectant mother, like the Natera Panorama, are now being used regularly to screen fetuses for chromosome abnormalities. At my wife's recent prenatal visit, she wasn't really even asked if she wanted the test; apparently it's standard in that office. I understand some of the science of how such a test could be performed. When the placenta implants, some of the fetal cells get into the mother's bloodstream, and even though the chromosomes from those cells are breaking down, these pieces can be scanned for the markers for genetic disorders and gender. But I don't understand how the fetus's maternal chromosomes can be identified against the background of the mother's genome.
For example, one of the things that they can rule out with pretty high certainty is Monosomy X: only one X chromosome and no Y. I can see that after determining the mother's genome, it could be determined if there was no evidence of an X or Y from the father. But how would they know if an X was not contributed by the mother. If they are looking through decomposing pieces of DNA in the mother's bloodstream, are they not guaranteed to find each of her X chromosomes, whether or not the fetus has a copy of either one? How can they determine that any piece of a maternal fetal chromosome really came from the fetus rather than from the mother?
Disclaimer: I do not endorse Natera over any of their competitors. I mention them by name only because they are the only ones I am familiar with.