Embryo screening, scientifically known as Preimplantation Genetic Testing (PGT), is an advanced procedure performed in conjunction with in vitro fertilization (IVF). This testing analyzes embryos for potential genetic or chromosomal irregularities before they are transferred into the uterus. PGT provides information about an embryo’s genetic makeup, allowing prospective parents and their medical team to make informed decisions about which embryos to proceed with for implantation. This process aims to increase the likelihood of a successful pregnancy and the birth of a healthy child.
Indications for Embryo Screening
Couples typically pursue embryo screening when specific risk factors are present that may compromise the success of an IVF cycle or increase the chances of a genetic disorder. One common reason is advanced maternal age, generally defined as the egg provider being 35 or older. The risk of producing eggs with an incorrect number of chromosomes significantly increases with age, and screening helps identify embryos with the correct chromosome count.
A history of reproductive challenges is another major indication, such as recurrent pregnancy loss. These losses are frequently caused by chromosomal abnormalities, and PGT offers a way to identify and avoid transferring affected embryos. Couples who have experienced multiple failed IVF cycles despite transferring morphologically healthy embryos may also opt for screening to pinpoint a hidden genetic cause for implantation failure.
For families with a known inherited condition, PGT offers a way to prevent the specific disorder from being passed on to the child. This is used when one or both parents are known carriers of a single-gene mutation or have a family history of a disease like cystic fibrosis or sickle cell anemia. PGT allows for the selection of embryos that are either unaffected or only carriers of the recessive gene.
Distinguishing Types of Genetic Testing
Embryo screening is categorized into different types based on the specific genetic material being examined. The most frequently performed test is Preimplantation Genetic Testing for Aneuploidy (PGT-A), which focuses on the number of chromosomes present in the embryo’s cells. PGT-A is essentially a “counting check” to ensure a complete set of chromosomes is present.
Embryos with missing or extra whole chromosomes are called aneuploid, and these are the primary cause of implantation failure and miscarriage. For example, PGT-A can identify a trisomy, where there is an extra copy of a chromosome, such as Trisomy 21 (Down syndrome). By screening for these numerical errors, the test aims to select euploid embryos, which have the correct number of chromosomes, for transfer.
In contrast, Preimplantation Genetic Testing for Monogenic disorders (PGT-M) is a highly specialized “spelling check” designed to look for a specific mutation within a single gene. This test is only performed when a precise, disease-causing genetic mutation is known to exist in the family. The laboratory must first create a custom-designed test, often taking several weeks, to target the exact gene sequence responsible for the condition.
A third category, PGT for Structural Rearrangements (PGT-SR), is used when one parent carries a change in the structure of their chromosomes, such as a translocation or inversion. This rearrangement can lead to the embryo having missing or extra pieces of chromosomal material.
The Preimplantation Screening Process
The process of embryo screening begins with the standard steps of an IVF cycle, including ovarian stimulation, egg retrieval, and fertilization. The resulting embryos are cultured for five to six days until they reach the blastocyst stage of development. At this stage, the embryo has differentiated into the inner cell mass (which becomes the fetus) and the trophectoderm (which forms the placenta).
The physical step of the screening is the trophectoderm biopsy. A small cluster of cells, typically between three and ten, is carefully removed from the trophectoderm layer using microscopic tools. This sampling avoids the inner cell mass, minimizing the risk of harm to the cells that will develop into the baby.
Immediately following the biopsy, the embryo is cryopreserved through a process called vitrification. This freezing step is necessary because the genetic analysis of the biopsied cells takes approximately one to two weeks to complete. The embryo is stored until the laboratory results are available and a subsequent transfer cycle can be planned.
The biopsied cells are sent to a specialized genetics laboratory where the DNA is amplified and analyzed using advanced techniques like Next Generation Sequencing (NGS). This technology allows for a detailed examination of the genetic material, either to count chromosomes for PGT-A or to check for a specific mutation for PGT-M. Once the final report is issued, the medical team selects the appropriate embryo for transfer.
Interpreting Test Outcomes
The results of embryo screening classify each embryo into categories that guide the decision to transfer. A “euploid” result means the embryo appears to have the correct number of chromosomes, indicating a higher chance of successful implantation. Conversely, an “aneuploid” result means an incorrect number of chromosomes was detected, and these embryos are typically not recommended for transfer.
For PGT-M, the outcome will indicate whether the embryo is “unaffected,” “a carrier” of the gene, or “affected” by the specific genetic disorder being tested. Carriers possess one copy of a recessive mutation but are not expected to develop the disease. Carriers are often considered for transfer alongside unaffected embryos.
A more complex result is “mosaic,” meaning the biopsy sample contained a mixture of both normal (euploid) and abnormal (aneuploid) cells. This complicates selection because it is unclear if the abnormal cells are confined to the future placental tissue or are also present in the cells that will form the fetus. The transfer of mosaic embryos is sometimes considered when no fully euploid embryos are available. However, they are associated with a slightly lower success rate and require careful counseling.
PGT is a screening test and not a definitive diagnosis. A small possibility of a false positive or false negative result exists. Patients who achieve pregnancy with a screened embryo are often advised to undergo further prenatal testing, such as amniocentesis, for confirmation.