In vitro fertilization (IVF) relies on careful laboratory evaluation to determine which embryos have the highest potential for a successful pregnancy. Embryo grading is the method embryologists use to predict an embryo’s developmental capacity and likelihood of implantation. This assessment is particularly important when deciding which embryos are suitable for cryopreservation, or freezing, for future use. The grades assigned help guide the selection process, ensuring that only the most robust specimens are preserved.
Understanding Embryo Development Stages in IVF
The timeline of development in the lab is a factor in how an embryo is graded for freezing. Embryos are typically evaluated at two main points: the Cleavage Stage (Day 3) and the Blastocyst Stage (Day 5 or Day 6). The criteria for assessment differ significantly between these two developmental milestones.
Day 3 embryos are graded based on cell number and fragmentation. A healthy Day 3 embryo typically contains six to ten cells, and lower fragmentation indicates better quality. The Blastocyst Stage is preferred for cryopreservation because it offers a more complex and predictive set of selection criteria.
By Day 5 or Day 6, a high-quality embryo has developed into a blastocyst, characterized by hundreds of cells and distinct cell lineages. Freezing is most often done at this later stage because the embryo has demonstrated the resilience necessary to survive several days of culture. This natural selection process helps ensure the frozen cohort consists of embryos with the greatest developmental potential.
The Morphology Grading System Explained
The Gardner Grading System is the most widely used method for assessing blastocysts, providing a detailed, three-part score. This alphanumeric system is applied to embryos that have reached the blastocyst stage, typically on Day 5 or Day 6. The first component is a number (1-6) representing the degree of expansion of the blastocyst cavity.
An expansion grade of 1 indicates an early blastocyst; 4 signifies a fully expanded blastocyst; and 6 is a fully hatched blastocyst. Following the number are two letters describing the quality of the two distinct cell groups. The first letter grades the Inner Cell Mass (ICM), which forms the fetus, and the second letter grades the Trophectoderm (TE), which develops into the placenta.
Both the ICM and TE are graded using the letters A, B, or C. A represents the highest quality, B is good or fair quality, and C is poor quality. For instance, a high-quality blastocyst might be graded as 5AA, meaning it is significantly expanded with excellent ICM (A) and TE (A). Conversely, a grade like 3BC indicates a moderately expanded blastocyst with a good quality ICM (B) but a poor quality trophectoderm (C).
A grade of A for the ICM signifies many tightly packed cells, while an A for the TE means many cells forming a cohesive layer. A C grade suggests very few cells or a disorganized appearance, indicating a lower probability of successful development. Understanding these three factors allows embryologists to rank embryos for selection, transfer, and cryopreservation.
Criteria for Embryo Cryopreservation
The decision to freeze an embryo is selective, focusing on its ability to survive the freeze-thaw process and potential for implantation. Clinics typically require a minimum expansion level of grade 3 or higher for blastocysts, meaning the fluid-filled cavity is fully formed. This expansion ensures the embryo is structurally mature enough to withstand the stress of cryopreservation.
Regarding cell quality, the most common guideline is to freeze embryos with an Inner Cell Mass and Trophectoderm grade of B or better (A or B). Therefore, a blastocyst graded 3AB, 4BB, or 5AA is considered an excellent candidate for freezing. Embryos with a C grade in either the ICM or TE, such as 4AC or 5CB, are often excluded because their lower cell quality presents a higher risk of not surviving the thaw.
The method of freezing plays a role in survival expectations, with Vitrification being the standard technique today. Vitrification is an ultra-rapid cooling process that minimizes the formation of ice crystals, which can damage the cell structure. This improved technology allows clinics to successfully freeze a broader range of high-quality embryos than was possible with older, slow-freezing methods.
Post-Thaw Viability and Implantation Potential
Selecting high-grade embryos for freezing is based on the direct correlation between pre-freeze morphology and post-thaw performance. High-grade blastocysts (A or B scores for both ICM and TE) demonstrate excellent post-thaw survival rates, often exceeding 90%. These embryos maintain structural integrity and re-expand after warming, which indicates viability.
The implantation potential of top-quality frozen-thawed embryos is often comparable to that of fresh embryos of the same grade. Lower-grade embryos, particularly those with a C grade, have significantly reduced post-thaw survival and implantation rates. If an embryo does not re-expand or shows significant cell damage after thawing, its chance of successful pregnancy is reduced.
This performance difference highlights why the strict grading process before freezing is important, as it ensures patients are storing embryos that offer a realistic chance of success. While some lower-grade embryos can result in a pregnancy, the clinical focus is on maximizing efficiency and outcomes by prioritizing embryos with the best predicted viability. Extended culture after thawing can sometimes reveal that a fair-quality embryo will develop to a better grade, positively impacting its pregnancy rate.