In Vitro Fertilization (IVF) involves the careful creation and selection of embryos. This process uses two key assessments: embryo grading, which evaluates visual appearance, and Preimplantation Genetic Testing (PGT), which analyzes genetic makeup. This article explores how these tools contribute to embryo assessment and selection, addressing whether embryo grading remains significant when PGT is also performed.
Embryo Grading Explained
Embryo grading is a visual assessment performed by embryologists to determine an embryo’s developmental potential based on its appearance. This evaluation typically occurs on day 3 and day 5 or 6 of development. On day 5 or 6, embryos are called blastocysts. Their quality is assessed using criteria like the Gardner grading system, which assigns scores to three components.
The system considers blastocyst expansion, indicating growth within its outer shell. It also evaluates the inner cell mass (ICM), the cluster of cells that will form the fetus, noting its size and compaction. The third component assessed is the trophectoderm (TE), the outer layer of cells that develops into the placenta. These visual characteristics indicate the embryo’s developmental progress and potential to implant. However, this assessment is purely morphological and does not reveal the embryo’s genetic health.
Preimplantation Genetic Testing Explained
Preimplantation Genetic Testing (PGT) is a diagnostic technique used with IVF to examine embryos for genetic conditions before transfer. PGT for aneuploidy (PGT-A) is the most common type, identifying chromosomal abnormalities. Aneuploidy refers to an incorrect number of chromosomes.
The process involves removing a small number of cells, typically from the trophectoderm layer of a day 5 or 6 blastocyst, in a biopsy. These cells are sent for genetic analysis to determine if the embryo has the correct number of chromosomes (euploid) or an abnormal number (aneuploid). PGT-A provides specific genetic information, distinct from morphological assessment.
The Interplay: How Grading and PGT Results Relate
While embryo grading and PGT-A are distinct, there is a correlation between an embryo’s morphological grade and its likelihood of being euploid. Higher-grade embryos tend to have a greater probability of a normal chromosomal count. Studies show excellent-grade embryos have higher euploidy rates compared to fair or poor-grade embryos.
This correlation is not absolute; discordance between morphology and genetic status can occur. A high-grade embryo can be aneuploid, appearing visually perfect but having chromosomal abnormalities. Conversely, lower-grade embryos can sometimes be euploid. For example, some reports indicate that even optimal morphological scores can include aneuploid embryos. PGT-A provides definitive genetic information regarding aneuploidy, which morphology alone cannot. An embryo’s appearance does not reveal its chromosomal integrity. Only genetic testing can confirm the correct number of chromosomes, identifying embryos less likely to implant or result in a healthy pregnancy due to chromosomal issues.
Informing Embryo Selection
Fertility clinics use both PGT results and morphological grade to decide which embryos to transfer. PGT results are the primary filter. Only euploid embryos, confirmed to have the correct number of chromosomes, are typically considered for transfer, as they are most likely to lead to a successful pregnancy and reduce miscarriage risk.
When multiple euploid embryos are available, morphological grading becomes a secondary factor to prioritize transfer. Higher-grade euploid embryos are generally preferred due to their greater potential for implantation. Research indicates that among euploid embryos, those with higher morphological grades, especially concerning the inner cell mass, are associated with higher ongoing pregnancy and live birth rates. If PGT results are inconclusive, such as with mosaic embryos, morphology can play a more prominent role. For patients who do not undergo PGT, morphological grading remains the primary method for embryo selection.
Key Takeaways for Patients
Preimplantation Genetic Testing (PGT-A) provides genetic information, making it the primary factor for assessing an embryo’s chromosomal health and viability. This testing directly addresses chromosomal abnormalities, which can impact implantation and pregnancy outcomes. Embryo grading still holds importance as a secondary factor.
Morphological grading helps evaluate an embryo’s developmental quality and implantation potential, especially when choosing among multiple euploid embryos. Fertility teams consider both PGT results and morphological grades to identify embryos with the highest likelihood of leading to a successful pregnancy. Patients can expect their care team to explain the specific grades and PGT results for their embryos, guiding them through selection.