Preimplantation Genetic Diagnosis or PGD has a margin of error of 1-2%, whether the test indicates that the chromosomes are normal or not. Therefore, although it is very rare, the situation can occur in which a PGT is normal and the embryo is actually affected by trisomy 21 or Down syndrome.
Fundamentally, this error occurs in cases in which there is mosaicism, that is, not all the cells of the embryo have 3 arms of chromosome 21. If the cells obtained in the biopsy are normal, the PGT will be normal. Depending on the proportion of affected and healthy cells, the diagnostic accuracy and the margin of error will depend on this.
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