Agreed, mostly, at least from first principles. The enrichment will be very modest. On average for any given chromosome, 45% of DNA in the mother's blood should

Agreed, mostly, at least from first principles. The enrichment will be very modest. On average for any given chromosome, 45% of DNA in the mother's blood should be maternal DNA from one chromosomal copy or the other. This gets us to 90%. The remaining 10% is split between one paternal copy and one maternal copy. This means that--if we ignore other differences such as fragment length--you're potentially trying to label the chromosomal copy that comprises 50% of DNA as the one inherited by the fetus, versus the one comprising 45% of DNA as the one not inherited by the fetus. That's a very small enrichment. Now, because of the length differences that they identified, they may be able to vastly enrich for fetal DNA (or precipitate out maternal DNA), obviating this problem.