In vitro fertilization (IVF) offers a pathway to parenthood. Embryo cryopreservation, or embryo freezing, is a common practice allowing storage of embryos for future use, increasing pregnancy chances. Not all embryos are suitable for freezing; embryo quality assessment determines which can survive freezing and thawing to lead to a successful pregnancy.
How Embryos Are Graded
Embryologists evaluate embryos at specific developmental stages to determine quality, a process known as embryo grading. This assessment typically occurs on Day 3 (cleavage stage) or Day 5/6 (blastocyst stage). Different grading systems are used for each stage due to their distinct characteristics.
At the cleavage stage (Day 3), embryos are assessed by cell number, uniformity (blastomeres), and cellular fragmentation. A healthy Day 3 embryo typically has six to ten cells with minimal fragmentation. Significant fragmentation indicates reduced developmental potential.
By Day 5 or 6, a good quality embryo develops into a blastocyst, characterized by hundreds of cells and a fluid-filled blastocoel. Blastocyst grading involves three components: expansion (rated 1-6), inner cell mass (ICM) quality, and trophectoderm (TE) quality. The ICM forms the fetus, while the TE develops into the placenta. Both are graded A to C (‘A’ best, ‘B’ good, ‘C’ fair). For instance, a “4AA” blastocyst indicates an expanded embryo with excellent ICM and TE quality.
Embryo Quality for Freezing
The decision to freeze an embryo is largely based on its morphological grade, prioritizing those with the strongest developmental potential. Typically, only good to excellent quality blastocysts are selected for cryopreservation due to their robust cellular structure and higher likelihood of surviving the freeze-thaw process and subsequently implanting.
For blastocysts, higher expansion grades (e.g., 3-6) combined with good ICM and TE quality (e.g., ‘A’ or ‘B’ grades) are generally considered suitable. Grades like 4AA, 4AB, or 4BA indicate a well-expanded blastocyst with healthy inner cell mass and trophectoderm, suggesting good freezing potential and a higher chance of successful development.
While Day 3 embryos can sometimes be frozen, many clinics culture embryos to the blastocyst stage first. This extended culture allows embryologists to observe further development and select only the most viable embryos, as blastocysts are inherently more robust. The advanced vitrification method, a rapid freezing technique, has significantly improved frozen embryo survival rates, particularly for blastocysts, by preventing the formation of damaging ice crystals.
Factors Beyond Grade in Freezing Decisions
While embryo grade is a significant determinant, other factors also influence the decision to cryopreserve embryos. The total number of viable embryos plays a role; if a patient has multiple high-quality embryos, some may be frozen even if slightly lower in grade than those for immediate transfer. This strategy helps maximize future pregnancy potential from a single egg retrieval.
A patient’s medical history and specific fertility challenges also influence these decisions. For instance, if a patient is at risk of ovarian hyperstimulation syndrome (OHSS), all viable embryos might be frozen to allow the body to recover before a transfer. If preimplantation genetic testing (PGT) is performed, embryos are typically biopsied at the blastocyst stage and frozen while awaiting results. Only genetically balanced embryos are then considered for transfer. Clinic-specific protocols and laboratory experience also guide these choices.
Outcomes of Frozen Embryo Transfers
Frozen embryo transfers (FETs) are a common and successful aspect of IVF treatment. Advances in cryopreservation, particularly vitrification, have led to high survival rates for embryos after thawing, typically 95% or higher for good quality blastocysts. This means most successfully frozen embryos will survive thawing and be suitable for transfer.
Many studies indicate FETs can have comparable or higher success rates than fresh embryo transfers. The uterine environment in a FET cycle is often more naturally receptive, as it is not influenced by high hormone levels from ovarian stimulation. This optimized uterine lining enhances implantation chances. Good quality frozen embryos maintain their developmental potential, offering a strong chance of successful pregnancy.