Genetic testing is integrated into the In Vitro Fertilization (IVF) process to improve the likelihood of a successful pregnancy and live birth. This testing allows specialists to evaluate the genetic health of the intended parents and the resulting embryos before implantation. The goal is to identify risks for inherited disorders or chromosomal abnormalities that commonly lead to implantation failure or miscarriage. Utilizing genetic data helps refine the selection process, prioritizing embryos with the greatest potential for healthy development.
Pre-IVF Genetic Screening
Genetic screening performed before an IVF cycle focuses on the intended parents to assess the risk of passing on specific inherited conditions. This is known as carrier screening, which determines if a parent carries a recessive gene mutation. Carriers are typically healthy because they possess one normal and one mutated copy of the gene. If both parents are carriers for the same autosomal recessive condition, there is a one in four chance the child will inherit the disorder. Panels can test for hundreds of conditions, including cystic fibrosis, spinal muscular atrophy, and sickle cell disease.
A simple blood or saliva sample is used for this analysis, ideally completed before the IVF cycle begins. Knowing the parents’ carrier status allows for informed decisions regarding the subsequent steps of the IVF process. If a couple is found to be at high risk, they may choose to utilize specific preimplantation genetic testing to screen the embryos themselves.
Testing Embryos for Chromosomal Health
The most common form of embryo testing screens for the correct number of chromosomes, a procedure known as Preimplantation Genetic Testing for Aneuploidy (PGT-A). Aneuploidy describes a state where an embryo has missing or extra chromosomes, such as three copies of chromosome 21, which causes Down Syndrome. Since a normal human cell contains 46 chromosomes (23 pairs), any deviation from this number is often incompatible with a successful pregnancy.
PGT-A is frequently recommended for patients with a history of recurrent miscarriage, multiple failed IVF cycles, or those of advanced reproductive age, as the incidence of aneuploidy increases significantly with age. The test aims to select embryos with a complete set of chromosomes, known as euploid embryos, which have a higher probability of implantation and healthy development. The technology uses next-generation sequencing to count the DNA representing each chromosome.
Sometimes, the test reveals a condition called mosaicism, where an embryo contains a mixture of both normal (euploid) and abnormal (aneuploid) cells. Mosaicism complicates the selection process because the small cell sample taken may not perfectly represent the entire embryo. Research indicates that embryos diagnosed as mosaic can still result in a healthy live birth, which has prompted ongoing research into the clinical management of these findings.
Testing Embryos for Single Gene Disorders
When parents are known carriers for a specific inherited disease, a highly customized test called Preimplantation Genetic Testing for Monogenic Disorders (PGT-M) is used to screen the embryos. Monogenic disorders are caused by a mutation in a single gene, such as Huntington’s disease or muscular dystrophy. PGT-M is designed to look for the exact familial mutation identified through the parents’ prior carrier screening.
This process requires a significant preparatory phase before the IVF cycle begins, often taking several weeks to develop a specific test, sometimes called a probe. This custom preparation involves analyzing DNA samples from both parents to create a unique genetic fingerprint associated with the mutation. The test then determines if an embryo is affected by the condition, is a healthy carrier, or is unaffected and non-carrier.
A related test, Preimplantation Genetic Testing for Structural Rearrangements (PGT-SR), is utilized when one parent has a known chromosomal structural abnormality, such as a translocation or inversion. While the parent with the rearrangement may be healthy, they have an increased risk of creating embryos with an unbalanced set of genetic material. PGT-SR specifically screens for these structural issues to reduce the chance of recurrent miscarriage or birth defects.
The Embryo Testing Procedure and Selection
The physical process for all types of preimplantation genetic testing begins once the embryos reach the blastocyst stage, typically on day five or six of development. At this stage, the embryo has differentiated into the inner cell mass (which forms the fetus) and the trophectoderm (which forms the placenta). An embryologist performs a biopsy by removing a small cluster of five to ten cells from the trophectoderm layer. This removal is carefully done to avoid disturbing the inner cell mass, minimizing risk to the future development of the fetus.
Following the biopsy, the embryos are flash-frozen using vitrification, ensuring their safety while the genetic testing is performed. The biopsied cells are sent to a specialized laboratory for analysis, which usually takes one to two weeks. Once the genetic results are returned, the fertility team identifies which embryos are euploid or unaffected by a specific single-gene disorder. The results guide the selection process, allowing the physician to choose the most genetically viable embryo for a subsequent frozen embryo transfer.