Preimplantation Genetic Testing

Definition And Nomenclature

Preimplantation genetic testing is a reproductive technology utilized alongside In Vitro Fertilization (IVF) to identify genetic defects in embryos prior to implantation.
It prevents the birth of an affected child and avoids the need for termination of pregnancy.
Preimplantation Genetic Diagnosis (PGD) and Preimplantation Genetic Screening (PGS) have been reclassified under the umbrella term Preimplantation Genetic Testing (PGT).
PGT-M (Monogenic) involves testing for single-gene defects.
PGT-SR (Structural Rearrangements) involves testing for chromosomal translocations, inversions, or deletions.
PGT-A (Aneuploidy) involves screening embryos for whole-chromosome aneuploidies.

The primary objective is to facilitate the birth of a healthy child free from a specific, known familial genetic disorder.
A secondary objective is to improve IVF success rates by transferring euploid embryos and reducing miscarriage rates associated with lethal chromosomal anomalies.
The core principle relies on the genetic equivalence of the biopsied cells to the rest of the embryo, assuming no significant mosaicism that would confound results.

Category	Specific Indications	Examples And Details
PGT-M	Autosomal Recessive Conditions	Spinal Muscular Atrophy, Cystic Fibrosis, Beta-Thalassemia, Sickle Cell Anemia; applicable when both parents are known carriers.
PGT-M	Autosomal Dominant Conditions	Huntington Disease, Neurofibromatosis Type 1, Tuberous Sclerosis Complex, Marfan Syndrome; applicable when one parent is affected or carries a de novo pathogenic variant.
PGT-M	X-Linked Conditions	Duchenne Muscular Dystrophy, Fragile X Syndrome, Hemophilia; applicable when the mother is a known carrier.
PGT-M	Non-Disclosure Testing	Late-onset dominant conditions where the at-risk parent desires disease-free offspring without knowing their own status.
PGT-SR	Structural Rearrangements	Reciprocal translocations, Robertsonian translocations, or inversions in one or both parents. History of recurrent miscarriages or prior birth of a child with an unbalanced karyotype.
PGT-A	Aneuploidy Screening	Advanced maternal age (typically over 35-37 years), recurrent implantation failure, recurrent pregnancy loss, severe male factor infertility.
HLA Typing	Savior Sibling	Selecting an embryo free of familial disease and Human Leukocyte Antigen (HLA) matched to an existing affected sibling to enable curative Hematopoietic Stem Cell Transplantation.

The exact genetic mutation or chromosomal aberration in the family must be unambiguously identified before offering testing.
Multidisciplinary genetic counseling is mandatory.
Informed consent must cover IVF risks, a 1-2% probability of misdiagnosis, lack of guaranteed pregnancy, and the possible need for confirmatory prenatal testing.
Customized probe design or linkage analysis panels must be developed specifically for the couple prior to initiating the IVF cycle.

Standard ovarian stimulation protocols are used to retrieve multiple oocytes.
Intracytoplasmic Sperm Injection (ICSI) is mandatory for all cycles to prevent DNA contamination from extraneous sperm attached to the zona pellucida, which could cause false amplification results.

Polar Body Biopsy (Day 0/1): Evaluates only maternal genetic contributions and is rarely used due to limitations.
Cleavage Stage Biopsy (Day 3): Involves removal of 1-2 blastomeres from a 6-8 cell embryo, carrying a high risk of mosaicism, lower DNA yield, and potential detrimental effect on embryo viability.
Blastocyst Stage Biopsy (Day 5/6): The current gold standard involves trophectoderm biopsy, removing 5-10 cells while leaving the inner cell mass intact. It provides a higher DNA yield, lower risk of mosaicism, and minimal impact on embryo developmental potential.

Whole Genome Amplification (WGA) is a critical prerequisite step to multiply the minute amount of DNA from biopsied cells to detectable levels.
Polymerase Chain Reaction (PCR) is used for PGT-M to detect point mutations, often combined with linkage analysis to prevent misdiagnosis due to allele drop-out.
Microarrays provide high-resolution detection of copy number variants, aneuploidies, uniparental disomy, and consanguinity.
Next-Generation Sequencing (NGS) is the preferred modality, offering high throughput, detection of segmental aneuploidies, and concurrent evaluation of single-gene defects and chromosomal status.
Karyomapping uses Single Nucleotide Polymorphism arrays as a universal linkage-based test, reducing the need for lengthy customized probe development.

Allele Drop-Out represents the failure of one allele to amplify during PCR, potentially causing a heterozygous embryo to appear falsely homozygous.
Trophectoderm mosaicism may not accurately reflect the inner cell mass.
Misdiagnosis occurs in 1-2% of cases due to allele drop-out, mosaicism, or technical errors.
Cycle cancellation can occur if all embryos are affected or fail to develop to the blastocyst stage.
Confirmatory prenatal diagnosis via Chorionic Villus Sampling or amniocentesis is strongly recommended for pregnancies achieved via testing.

Ethical dilemmas involve adult-onset conditions, savior siblings, and concerns regarding eugenics or designer babies.
The Pre-Conception and Pre-Natal Diagnostic Techniques (PCPNDT) Act strictly prohibits sex selection, forbidding testing for family balancing.
The Assisted Reproductive Technology (Regulation) Act mandates clinic registration and regulates genetic testing in embryos strictly for preventing heritable diseases.