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The accuracy of this testing depends heavily on which laboratory method is used, which is an important question for a couple to ask their clinic directly. The two older, more widely available methods, fluorescence in situ hybridization and array comparative genomic hybridization, can reliably detect an unbalanced embryo, but neither one can distinguish a completely normal embryo from an embryo that carries the same balanced rearrangement as the parent; both are simply reported as balanced. This distinction generally does not affect the health of a resulting child, since a balanced carrier is expected to be as healthy as any other person, but it does mean that a child conceived from such an embryo could grow up to carry the same rearrangement and face the same reproductive considerations as the parent did. Newer single nucleotide polymorphism array methods, which track inherited genetic marker patterns alongside the rearrangement, can make this distinction, so couples who wish to know whether a transferred embryo is fully normal or a balanced carrier should confirm this capability is available before testing begins.
Every method also has a detection limit. Very small rearranged segments, particularly those near the very ends of chromosomes, can fall below the resolution of array-based methods and be missed. Mosaicism, where a biopsy sample contains a mix of normal and abnormal cells, adds further complexity; newer sequencing-based methods are generally more sensitive at picking this up than older array methods, but a mosaic result can still be difficult to interpret with certainty, and taking a slightly larger cell sample from the trophectoderm, rather than a single cell, improves reliability while introducing a small chance of a result that does not perfectly reflect the rest of the embryo.
Because of these limitations, professional guidance continues to recommend that a pregnancy resulting from an embryo classified as balanced or normal still be offered confirmatory testing, through chorionic villus sampling or amniocentesis, once the pregnancy is established. Testing a full pregnancy sample provides a much higher level of certainty than testing a handful of cells from an early embryo, and this step remains the safety net that makes the overall approach dependable.
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