Why do IVF labs grade embryos?

1. To select the best embryo to transfer
   When you have multiple embryos, grading helps your fertility team decide which embryo has the highest likelihood of successful implantation and a healthy pregnancy. Modern practice increasingly favors single embryo transfer of the highest-grade embryo to reduce risks of multiples.[2]
   
   
2. To decide which embryos are suitable for freezing (vitrification)
   Not all embryos survive the freeze-thaw process equally well. Higher-grade embryos generally tolerate vitrification (ultra-rapid freezing) better. Embryos that don't meet minimum quality thresholds may not be recommended for cryopreservation.
   
   
3. To enable trophectoderm biopsy for preimplantation genetic testing
   If you're doing genetic testing (PGT-A or PGT-M), the embryo must reach the blastocyst stage (day 5 or later) and have sufficient trophectoderm cells for biopsy. Embryo grading helps identify which embryos are candidates for this procedure.[3]
   
   Important context: Embryo grading is somewhat subjective. Two embryologists might assign slightly different grades to the same embryo. However, most IVF clinics participate in proficiency testing programs and follow standardized grading systems (like the Gardner system for blastocysts) to ensure consistency.[4]

Cell Number (The Best Indicator)

Cell number indicates the rate of embryo development. After 3 days, a healthy embryo should have divided into approximately 6-10 cells (ideally 8 cells on day 3).[5]

What cell number tells us:

• Fewer than 6 cells: Slower development (lower potential)
• 6-10 cells: Normal, on-track development (good potential)
• More than 10 cells: Faster development (can be good, but rapid division sometimes indicates abnormalities)

Cell number is the single most predictive factor at day 3. An 8-cell embryo with slight fragmentation generally has better potential than a 5-cell embryo with perfect symmetry.

5AA — A hatching blastocyst with an excellent inner cell mass and excellent trophectoderm. This is considered a high-quality embryo with the highest potential for implantation and healthy pregnancy. Success rate: 60-70% per transfer.[1]

4BB — An expanded blastocyst with good ICM and good TE. This is a solid, high-quality embryo. Many healthy babies come from 4BB embryos. Success rate: 40-50% per transfer.[1]

4AB or 4BA — An expanded blastocyst with one component excellent (A) and one good (B). Still very strong. Success rate: 45-55% per transfer.

3BC — A full blastocyst with good ICM (B) but fair TE (C grade). This embryo has potential, especially if the ICM is strong, but the weaker TE suggests slightly lower odds. Success rate: 25-35% per transfer.[12]

3CC — A full blastocyst with both ICM and TE rated fair. Lower potential, but not hopeless. Some clinics may not freeze these; others will transfer if no better options exist. Success rate: 10-20% per transfer.

6AA — A fully hatched blastocyst with top grades. This sounds perfect, but embryos that hatch prematurely (too fast) sometimes have slightly lower success rates than grade 5AA because premature hatching can indicate chromosomal issues.[13]

1. Morphology ≠ Genetics
   Embryo grading is a tool based on appearance. But what you can't see under the microscope are chromosomal abnormalities (aneuploidy). A beautiful 5AA blastocyst might be chromosomally abnormal and fail to implant or miscarry. Conversely, a 4BC embryo might be chromosomally normal and result in a healthy baby.[14]

This is why preimplantation genetic testing (PGT-A) is increasingly common. PGT-A involves trophectoderm biopsy at the blastocyst stage, sending cells for chromosomal analysis. A chromosomally normal (euploid) embryo—even if it's graded 4BB—has better odds than an abnormal (aneuploid) 5AA embryo.[15]

3. Many Factors Beyond the Embryo Affect Success
   Successful pregnancy depends on:

• Uterus receptivity (endometrial lining quality, timing of transfer)
• Maternal age (impacts egg quality and chromosomal health)
• Underlying fertility issues
• Immune factors
• Luck

Even with a perfect embryo, success is never 100%.

If you're doing preimplantation genetic testing, the embryo grade becomes secondary to chromosomal status. Studies show that euploid (normal) embryos of grade 4BB have similar success rates to euploid embryos of grade 5AA—both around 60-65%.[17]

PGT-A involves:

1. Culturing embryos to the blastocyst stage (day 5 or day 6)
2. Performing trophectoderm biopsy (removing 5-10 cells from the TE layer)
3. Freezing embryos via vitrification while cells are sent for analysis
4. Transferring only euploid embryos in a subsequent frozen ET cycle

PGT-A is especially recommended for women over 37, recurrent miscarriage, or repeated IVF failure, as chromosomal abnormalities increase with maternal age.[18]

Blastocyst: An embryo at day 5-7 of development, consisting of 100-200 cells with a fluid-filled cavity (blastocoel) and distinct inner cell mass and trophectoderm layers.

Inner Cell Mass (ICM): The cluster of cells inside the blastocyst that will develop into the fetus.

Trophectoderm (TE): The outer layer of the blastocyst that will form the placenta and supporting structures.

Fragmentation: Cellular debris or fragments between cells in an embryo, often a sign of cellular stress or abnormal development.

Vitrification: Ultra-rapid freezing technique used to preserve embryos for future use with minimal ice crystal formation.

Trophectoderm Biopsy: Removal of 5-10 cells from the trophectoderm layer of a blastocyst for genetic testing (PGT).

Euploid: An embryo with the correct number of chromosomes (46 in humans).

Aneuploid: An embryo with an abnormal number of chromosomes (too many or too few).

Embryo grading has relied on human observation for decades, but the future is increasingly digital. Artificial Intelligence (AI) and machine learning are beginning to transform embryo quality assessment in IVF labs worldwide.[19]

How AI helps:

1. Time-lapse imaging: Some IVF clinics, including Next Generation Clinic St. Petersburg, use time-lapse incubators that take photos of embryos every 10-15 minutes. AI algorithms analyze thousands of images to detect patterns invisible to the human eye—such as the exact timing of cell divisions, which correlates with implantation success.
   
   
2. Objective grading: AI reduces subjectivity. Whether an embryo is graded BB or AB might vary between embryologists, but AI applies consistent criteria across all embryos.
   
   
3. Predictive power: AI models trained on thousands of IVF cycles can predict likelihood of successful implantation and live birth more accurately than morphology alone.[20]

Important reassurance: AI is a tool to assist embryologists, not replace them. Human expertise in embryo development, patient history, and clinical judgment remains essential. Think of AI as a second opinion that helps your fertility team make even better decisions about which embryo to transfer.