Preimplantation genetic testing (PGT)

Preimplantation genetic testing (PGT) is the analysis of embryos generated through in vitro fertilization (IVF) before transfer to the uterus. It allows couples at known risk of transmitting a genetic disease, or at increased aneuploidy risk, to select unaffected/euploid embryos. PGT requires IVF even for otherwise fertile couples.

• PGT-M (monogenic): tests for a specific known single-gene disorder (e.g., cystic fibrosis, Huntington disease, BRCA1/2). Requires prior identification of the familial variant and, usually, a family linkage workup to design the custom assay.
• PGT-SR (structural rearrangements): tests for unbalanced chromosome segregants in carriers of balanced reciprocal translocations, Robertsonian translocations, or inversions.
• PGT-A (aneuploidy): screens for whole-chromosome aneuploidies in embryos from couples without a specific known genetic risk. Historically called PGS (preimplantation genetic screening).

• Day 3 (blastomere biopsy): one cell from an 8-cell embryo. Largely abandoned because it reduces implantation potential and has high mosaicism-related error.
• Day 5–6 (trophectoderm biopsy): 5–10 cells from the outer trophectoderm of a blastocyst, leaving the inner cell mass (future fetus) untouched. Current standard.
• Polar body biopsy: samples the first and second polar bodies to infer maternal contribution. Used in some European programs for maternal mutations; doesn't detect paternal contribution or post-zygotic events.

• PGT-M: targeted PCR with multiplex analysis of the pathogenic variant plus flanking STR markers (haplotype tracking to detect allele drop-out). Some programs use karyomapping (SNP-based haplotyping).
• PGT-SR: array CGH or NGS to detect unbalanced segments.
• PGT-A: whole-genome low-pass NGS or array CGH to call chromosome-level copy number.

• Mosaicism: the biopsied trophectoderm may not reflect the inner cell mass. PGT-A calls of mosaicism (20–80% aneuploid reads) represent a genuinely uncertain category; some mosaic embryos transferred result in healthy euploid births.
• Allele drop-out: a heterozygous embryo may appear homozygous due to amplification failure. Haplotype-based designs mitigate this.
• Does not detect: de novo mutations not previously identified, uniparental disomy (unless specifically analyzed), imprinting disorders, structural variants below the resolution threshold.
• Confirmatory prenatal testing (CVS or amnio) is recommended for high-risk results even after a "normal" PGT, because no embryo biopsy eliminates risk completely.

• PGT-A benefit is age-dependent. Strongest benefit in women 35–42 where aneuploidy drives most implantation failure and miscarriage. Mixed evidence in younger women.
• Emotional and financial costs of IVF are substantial. Counseling should discuss alternatives: natural conception with prenatal diagnosis, donor gametes, adoption.
• HLA matching PGT is possible for couples seeking a savior sibling, permitted in some jurisdictions with ethics review.
• PGT for late-onset conditions (Huntington, BRCA) raises ethical discussion about discarding embryos that would develop into adults with normal life expectancy for decades.