Can PGT-A be wrong?

Yes, in both directions, and the reasons are worth understanding. An embryo reported as abnormal may not be. An embryo reported as normal may still have a chromosome problem. The underlying reason is that the biopsy takes a few cells from the outer layer — the part that becomes the placenta — not from the part that becomes the baby. Those cells usually reflect the embryo. Sometimes they do not. Specific causes ESHRE identifies: Mosaicism, where different cells have different chromosomes, may lead to misinterpretation of the embryo's actual chromosome makeup. Contamination of the sample, for example with cells left over from the egg collection. Cells damaged during biopsy, which can affect the reliability of the result. Limits of the technology itself — it cannot detect uniparental disomy, where both copies of a chromosome come from one parent, and it cannot always determine whether an error arose in the egg or sperm or after fertilisation. Because of all this, ESHRE recommends prenatal diagnosis is offered to every woman who becomes pregnant after PGT. The practical implication: treat a PGT-A result as good information rather than as certainty. Ask your clinic for their own misdiagnosis rate — ESHRE recommends this be available to you on request — and take up the offer of prenatal testing, or arrange cord blood confirmation at birth.