A fetus can now be screened for some of the more common genetic diseases and its sex determined from just a whole blood sample from the pregnant mother. Fetal DNA is abundant enough in the systemic circuit to be able to be purified and sequences analyzed. Question is, in the case of fraternal twins, could the procedure discern between the two fetus's...

As a clearer example, can it distinguish between:

Scenario 1: if one fetus was male and one fetus female And scenario 2: both male?

It appears the test would just look for presence of Y chromosome using certain markers. But it could then look at SNP to distinguish individuals.

Looking for clarification here


1 Answer 1


I think you are overestimating how these cfDNA tests work. Maybe I read the paper wrong, but from what I read of one technique (Sequenom's MaterniT21 test) there is no purification of fetal DNA. They do restriction digests to assess the percentage of fetal DNA, but I don't think they enrich for the fetal DNA when they sequence. The library they make is mostly maternal, they are just looking for slight deviations in how many reads come from each chromosome, if too many come from chromosome 21, they conclude it's because the fetus has too many copies.

And as for SNPs, coverage is way to low for that. We're talking about 40 million reads, even if that were mostly fetal DNA, which I don't think it is, that's not enough coverage to call SNPs.

So if there were two fetuses, and they were not identical, then the signal from each would be half what would be expected based on the restriction digests assay. So maybe you would have enough counts that you could tell that the Y chromosome counts were too low, and the X chromosome a bit too high, but since these tests almost certainly have not been clinically tested on multiple fetuses well enough for verify their accuracy, no company is going to report results.

  • $\begingroup$ qiagen.com/us/products/catalog/sample-technologies/… $\endgroup$
    – rhill45
    Commented Jan 4, 2015 at 14:40
  • $\begingroup$ My wife just had this test done and we are having twins. So the test is largely performed. Y chromosome was detected in her peripheral blood sample. The question I'm asking here is how could test be modified to allow identification-we should be able to descent if we are having two boys or one boy and one girl $\endgroup$
    – rhill45
    Commented Jan 4, 2015 at 14:46
  • $\begingroup$ Like I said, the signal from each fetus would be less. You'd need to boost that at least two fold, likely by increasing the real estate on the flow cell, which would cost more. And if they appeared to be the same sex, there would be no way to know if you had enough signal from both. $\endgroup$
    – swbarnes2
    Commented Jan 4, 2015 at 15:23
  • $\begingroup$ I know they use that kit, its the rest of the protocol that matters. $\endgroup$
    – swbarnes2
    Commented Jan 4, 2015 at 15:25

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