Just how the screening examination identifies sex
This test analyses the DNA of the placental cells. Detection of a Y chromosome is consistent with the visibility of a male foetus. Therefore, if no Y chromosome piece is discovered, the inferred sex is women.

In cases of twin maternities, if there is a Y chromosome present, we can establish that at the very least among the foetuses is male, and if there is no Y chromosome, we can think that both foetuses are women.

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