Preimplantation genetic testing (PGT) can be performed on embryos that have already been frozen. This common practice in fertility treatments assesses the genetic health of embryos before transfer, aiding in the selection of those most likely to lead to a successful pregnancy.
Understanding PGT and Embryo Freezing
Preimplantation genetic testing (PGT) identifies genetic conditions or chromosomal abnormalities in embryos created during in vitro fertilization (IVF). Its purpose is to select embryos free of specific genetic conditions or chromosome abnormalities for transfer. This aims to reduce the risk of genetic diseases and improve pregnancy success by identifying embryos less likely to implant or result in miscarriage.
There are three main types of PGT:
- PGT-A (for aneuploidy) screens for an atypical number of chromosomes, a common cause of implantation failure and miscarriage.
- PGT-M (for monogenic disorders) identifies specific single-gene defects like cystic fibrosis or sickle cell anemia.
- PGT-SR (for structural rearrangements) detects chromosomal structural abnormalities, such as translocations or inversions.
Embryo freezing, or cryopreservation, preserves embryos for future use. Embryos are often frozen to await genetic testing results, for subsequent transfer cycles, or for fertility preservation before medical treatments. Vitrification, a rapid freezing process, is the most common and successful method used today. It prevents damaging ice crystal formation and significantly improves embryo survival rates upon thawing.
The Process of Testing Frozen Embryos
Performing PGT on frozen embryos involves specific steps. The initial step is thawing the cryopreserved embryos. Vitrification techniques result in high survival rates, often around 98%, after thawing. Once thawed, embryos are cultured briefly to ensure viability and resumed development.
Following thawing and culture, a small number of cells are removed from each embryo. This procedure, known as a trophectoderm biopsy, is performed on day 5 or 6 of embryo development when the embryo has reached the blastocyst stage. The trophectoderm is the outer layer of cells that will eventually form the placenta, and removing cells from this region is considered less invasive to the inner cell mass, which develops into the fetus. An embryologist uses micromanipulation tools to create a small opening in the embryo’s outer shell (zona pellucida) and remove about five to ten cells.
After the biopsy, embryos are re-frozen using vitrification. This second cryopreservation step occurs while the biopsied cells are sent to a laboratory for genetic analysis. Genetic testing takes one to two weeks for results. Once results are available, normal embryos are selected for a frozen embryo transfer (FET) cycle. This transfer occurs in a separate cycle, allowing optimal patient preparation.
Important Considerations for Patients
Despite improved success rates with modern vitrification, a small risk of embryo damage exists during thawing, biopsy, or re-freezing. Studies show 91.5% to 98.9% of embryos survive thawing, with re-freezing also having high survival rates. However, some research suggests a potential decrease in live birth rates for embryos undergoing multiple freeze-thaw cycles compared to those biopsied and frozen only once.
Performing PGT on frozen embryos allows results to be obtained before transfer, allowing for a more planned transfer. This approach permits selecting a single, euploid embryo, which improves implantation rates and reduces the risk of multiple pregnancies and miscarriages. PGT does not guarantee a successful pregnancy or a healthy child, as it is a screening tool.
The expertise of the fertility clinic and its embryology laboratory is crucial. Clinics with experienced embryologists and robust cryopreservation programs are essential for handling the delicate procedures of thawing, biopsy, and re-freezing. The cost of PGT on frozen embryos must also be factored into financial planning. PGT-A costs approximately $350-$500 per embryo for testing, but total costs including biopsy, shipping, and laboratory fees often range from $4,000 to $10,000 per IVF cycle for PGT services. These costs are in addition to standard IVF cycle expenses.