Preimplantation Genetic Testing (PGT)
Preimplantation Genetic Testing (PGT)can be used to evaluate embryos for certain types of genetic abnormalities before they are transferred into the uterus to attempt pregnancy. Embryos are first created through the process of in vitro fertilization (IVF), and grown for 5-7 days in the laboratory until they reach the blastocyst stage of development. At this point, an RMA embryologist uses a laser to take a small biopsy of a few cells from the outer layer of the embryo (the part of the embryo that becomes the placenta). After biopsy, the embryos are frozen and the biopsies are sent to an outside laboratory specializing in PGT for analysis. Test results typically take 1-3 weeks, depending on the type of testing being performed. Once results are received, an embryo transfer cycle can be scheduled. The choice of which embryo to use for transfer is made by consulting your physician and/or a genetic counselor to help interpret the results.
RMA of New York offers three types of PGT:
- PGT-Aneuploidy (PGT-A)
- PGT-Monogenic Conditions (PGT-M)
- PGT-Structural Rearrangements (PGT-SR)
Preimplantation Genetic Testing for Aneuploidy (formerly known as Preimplantation Genetic Screening, or PGS) is used to test embryos for chromosome abnormalities. Chromosomes are the structures inside our cells that hold all our genes—the information that tells our bodies how to work properly. Humans have 23 pairs of chromosomes (for a total of 46) and receive one set of chromosomes from each parent - 23 chromosomes in the oocyte (egg) cell, and 23 in the sperm cell. However, when our bodies create eggs and sperm random errors can occur that cause them to have extra or missing chromosomes—a phenomenon also known as ‘aneuploidy’. Chromosome abnormalities can occur randomly in the egg or sperm cells of an individual at any age, but do occur more frequently in eggs cells as age increases.
The human body is very particular about needing exactly 46 chromosomes in 23 pairs, so having any extra or missing chromosomes is usually not compatible with healthy human development. If pregnancy is attempted with a chromosomally abnormal embryo, typically that embryo will either fail to implant in the uterus (resulting in no pregnancy that month) or implant but then result in a pregnancy loss. Beyond that, certain chromosome abnormalities can result in a live birth with physical or intellectual disabilities. Whether an embryo has the correct number of chromosomes (euploid), an incorrect number of chromosomes (aneuploid), or is mosaic (a mixture of euploid and aneuploid cells) is not outwardly visible when observing the embryos in the IVF laboratory. However, PGT-A can be used in order to identify healthy embryos for transfer, reducing the risk of pregnancy loss, failed implantation, or a chromosomally abnormal pregnancy in comparison to treatment without PGT-A - helping patients achieve their family-building goals with fewer setbacks along the way.
Monogenic Conditions (PGT-M)
Preimplantation Genetic Testing for Monogenic Conditions (PGT-M) is used to identify embryos at increased risk for specific monogenic disorders (genetic disorders causes by mutations, or changes, in a single specific gene), with the goal of only attempting pregnancy with embryos that are not at increased risk for the disorder of concern. This is available when a family is known to be at risk for a specific genetic disorder because of carrier screening, previous genetic testing, or a known family history.
Prior to pursuing PGT-M, the patient/couple must have undergone genetic testing already to determine the exact disorders for which their embryos are at risk AND the exact causative mutation (or gene change). Once that information is known, PGT-M typically requires several months of preparatory lab work before a PGT lab is ready to analyze samples. This is because PGT-M requires the creation of a custom test for the family (sometimes referred to as a “probe”) that is specific to the genetic mutation(s) involved and other identifying genetic markers. Patients/couples planning for PGT-M have the option of including concurrent PGT-A.
Structural Rearrangements (PGT-SR)
Structural rearrangements refer to when an individual has the correct number of chromosomes, but there are small segments of chromosomes that are duplicated, deleted, or rearranged. Some structural rearrangements cause disease, while others are major contributors to risk of pregnancy loss by making an individual more prone to creating embryos with extra or missing chromosome segments. Preimplantation Genetic Testing for Structure Rearrangements (PGT-SR) is used to identify embryos with the correct amount of chromosome material present for transfer. PGT-SR is highly case-specific in terms of which technology is most appropriate and whether any specialized preparation with a PGT lab ahead of time will be necessary. The choice of which technology can best detect the abnormality present will be determined by your team in conjunction with a genetics specialist.
Limitations of PGT
Like any form of testing, PGT has limitations which are important to keep in mind. While detection rates are high (approximately 97-99% depending on the type of PGT) they are not 100%, and there are small but present risks of both false positive and false negative results. In addition, certain rare types of chromosome abnormalities may not be detected by PGT-A/SR, and PGT-M cannot screen for genetic conditions outside of those for which the probe is targeted. In addition, PGT is unable to screen for conditions that do not have a specific, identifiable genetic cause (such as most cases of autism, cerebral palsy, or spontaneous birth defects). Because of the limitations of PGT, follow up prenatal genetic testing should still be considered and further discussion of prenatal testing options with a prenatal genetic counselor and/or OB/GYN is recommended.
“Optimal care of the infertile couple involves compassion, science, and data-driven decisions.”
Why RMA of New York: Preimplantation Genetic Testing
RMA of New York offers patients personalized, compassionate, and premium fertility care. As the Division of Reproductive Endocrinology and Infertility of Mount Sinai Hospital, our team is dedicated to scientific discovery, graduate medical education, and patient-focused care. We are proud to be a member of the Department of Obstetrics, Gynecology, and Reproductive Science at the Icahn School of Medicine at Mount Sinai and of recognition as a Center of Excellence by US News & World Report.
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