Embryo Grading & PGT-A — What the Grades Mean for IVF Success

Why Embryo Selection Matters

Not every embryo becomes a baby — and that is the central clinical reality every IVF patient needs to understand before their retrieval cycle. Embryo selection is the process by which embryologists evaluate the quality of the embryos that develop in the lab and decide which ones offer the best chance of implantation and a live birth. The better you understand this process, the more productive your conversations with your care team will be, and the better equipped you'll be to interpret news — good or difficult — about your embryo cohort.

The numbers tell the story. According to SART (Society for Assisted Reproductive Technology), the average stimulated IVF cycle retrieves 5 to 15 eggs, of which roughly 70–80% are mature enough to fertilize. Of those mature eggs, 60–75% will fertilize successfully. But here's where the attrition becomes significant: of all the fertilized eggs (zygotes) that begin developing, only 30–50% will reach the blastocyst stage by Day 5 or Day 6 — the stage at which most clinics prefer to transfer or freeze embryos.

What happens to the rest? Some embryos arrest in development due to chromosomal abnormalities, poor egg or sperm DNA quality, or suboptimal conditions in the lab. This natural attrition is not a failure of your fertility treatment — it is biology filtering out embryos that were unlikely to produce a viable pregnancy in the first place.

Understanding this funnel helps explain why embryo grading and genetic screening exist: with a limited number of embryos reaching blastocyst stage, choosing the right one to transfer first can be the difference between a successful pregnancy and another transfer attempt.

Day 3 (Cleavage Stage) vs. Day 5 (Blastocyst) Embryos

For much of IVF's early history, embryos were transferred on Day 3 of development, when they had divided into 6 to 10 individual cells. This was called the cleavage stage. Embryologists would evaluate these embryos based on cell number, symmetry of the cells, and the degree of cellular fragmentation — small anucleate fragments that break off during division and indicate developmental stress.

Day 3 grading provided useful information, but it had a fundamental limitation: a large proportion of cleavage-stage embryos that looked good under the microscope would fail to implant or miscarry, often because they harbored chromosomal abnormalities that couldn't be identified at that stage. The embryo's own genome doesn't fully activate until around Days 3–4, meaning earlier development is driven largely by maternal proteins inherited from the egg. Only after the embryonic genome activates do the consequences of chromosomal errors become visible in the embryo's development.

This is why extending culture to Day 5 or 6, when the embryo reaches the blastocyst stage, has largely replaced Day 3 transfer at most high-volume IVF centers. A Cochrane systematic review of randomized controlled trials found that blastocyst-stage transfer improves the implantation rate per transfer compared to cleavage-stage transfer. Blastocysts that survive to Day 5 have already demonstrated the capacity for sustained development, which makes them more predictive of implantation potential. Only embryos that tolerate culture long enough to reach blastocyst stage are likely to contain at least some normal cells.

That said, Day 3 transfer still has a role. Patients with very few fertilized eggs may not benefit from extended culture, because the attrition from Day 3 to Day 5 that happens in the lab would also happen in the uterus — and the uterine environment may actually support some marginal embryos better than any culture system. Some clinics with specific laboratory protocols also prefer earlier transfer in certain circumstances. Your embryologist and reproductive endocrinologist will make this call based on your specific cohort.

Blastocyst Grading — The Gardner Scale

The most widely used system for evaluating blastocyst-stage embryos is the Gardner grading system, developed by reproductive scientist Dr. David Gardner. It evaluates three independent parameters and combines them into a grade that communicates overall embryo quality. Understanding each component helps you make sense of the grades your clinic reports.

1. Expansion Stage (Graded 1–6)

The first number in a blastocyst grade describes how far the embryo has expanded. As the blastocyst develops, a fluid-filled cavity called the blastocoel enlarges, eventually causing the embryo to expand and thin its outer shell (the zona pellucida) until it hatches.

Grades 4, 5, and 6 represent the most advanced blastocysts and are generally preferred for biopsy (if PGT-A is being performed) and for transfer or vitrification.

2. Inner Cell Mass (ICM) — Graded A, B, or C

The first letter in a blastocyst grade describes the inner cell mass, a compact cluster of cells nestled inside the blastocoel. This cluster of cells is critically important: it is the ICM that will eventually develop into the fetus itself. The trophectoderm cells surrounding it will become the placenta and supporting structures.

An ICM grade of A is associated with the best clinical outcomes. When embryologists report ICM quality, they are directly commenting on the developmental potential of the cells that will become your baby.

3. Trophectoderm (TE) — Graded A, B, or C

The second letter in a blastocyst grade describes the trophectoderm — the outer layer of cells forming a shell around the blastocoel. These cells will differentiate into the placenta and are responsible for implantation into the uterine lining.

Reading a Combined Blastocyst Grade

A blastocyst grade is written as a number followed by two letters — for example, 4AA, 3BB, or 5AB. The number is the expansion stage, the first letter is the ICM quality, and the second letter is the trophectoderm quality.

A 4AA embryo is expanded (4), has a tightly packed inner cell mass (A), and a cohesive, well-organized trophectoderm layer (A). This is the highest quality designation in most grading systems.

Approximate implantation rates by grade (for reference — your clinic's specific data may differ):

Research published in Fertility and Sterility — including work by Ahlstrom and colleagues — has confirmed that both ICM and trophectoderm grades independently predict implantation success at blastocyst transfer, with combined high-grade embryos significantly outperforming lower-grade counterparts.

An important caveat on grading subjectivity: Embryo grading is performed visually under a microscope by a trained embryologist, and like any human assessment, it carries inter-observer variability. Two embryologists looking at the same blastocyst may assign slightly different grades, particularly at borderline quality levels. When you're making decisions about your embryos, ask your clinic what standardization practices they use and whether embryologists calibrate their grading against each other regularly. This matters more than you might expect.

What Is PGT-A?

Preimplantation Genetic Testing for Aneuploidies — PGT-A — is a laboratory procedure that screens embryos for chromosomal abnormalities before transfer. It was previously called PGS (preimplantation genetic screening), and you may encounter both terms in older literature and at some clinics.

Chromosomal abnormalities — called aneuploidies — are the leading cause of implantation failure and early miscarriage in IVF cycles. An aneuploid embryo has the wrong number of chromosomes: either extra copies (trisomy) or missing copies (monosomy) of one or more chromosome pairs. Humans have 23 pairs of chromosomes, for a total of 46. An embryo with 47 or 45 chromosomes is aneuploid and will almost universally fail to implant or will miscarry, often very early.

How PGT-A Works

1. Blastocyst development: The embryo is cultured until it reaches the blastocyst stage, typically Day 5, 6, or sometimes Day 7.
2. Trophectoderm biopsy: The embryologist uses a fine laser and micropipette to remove 5 to 10 cells from the trophectoderm — the outer layer. Critically, the inner cell mass (the future baby) is not touched.
3. Vitrification: The biopsied embryo is immediately frozen (vitrified) while the genetic analysis is performed.
4. Genetic analysis: The biopsied cells are sent to a certified genetics laboratory, which uses next-generation sequencing (NGS) to evaluate all 23 chromosome pairs simultaneously.
5. Results: Typically available within 1 to 2 weeks. Results classify each embryo as euploid, aneuploid, or mosaic.

Understanding Your PGT-A Results

The American Society for Reproductive Medicine (ASRM) provides detailed guidance on interpreting PGT-A results and counseling patients. Consulting that document alongside your clinic's recommendations is valuable if you want a deeper clinical perspective.

What PGT-A Detects

PGT-A is designed to detect whole-chromosome and large segmental abnormalities. Common findings include:

• Trisomies (an extra chromosome): Trisomy 21 (Down syndrome), Trisomy 13, Trisomy 18, and many others that do not result in recognized syndromes but will cause miscarriage or failed implantation
• Monosomies (a missing chromosome): Turner syndrome (45,X) is the most viable monosomy; most others result in early pregnancy loss
• Segmental abnormalities: Large deletions or duplications of chromosome segments, some of which can have clinical significance

PGT-A does not detect single-gene disorders (those require PGT-M, or preimplantation genetic testing for monogenic disorders), and it does not evaluate the complete DNA sequence of the embryo. It is a chromosomal screening tool, not a comprehensive genetic diagnosis.

Does PGT-A Improve IVF Outcomes? The Evidence Debate

This is one of the most actively debated questions in reproductive medicine today, and it deserves an honest answer rather than a promotional one. The short version: PGT-A clearly helps some patients and may offer little benefit — or even harm — for others. The key is identifying which group you're in.

The Evidence Supporting PGT-A

The strongest argument for PGT-A is its ability to raise the implantation rate per transfer by selecting only chromosomally normal embryos. When only euploid embryos are transferred, implantation rates of 60–70% per transfer have been reported at high-volume centers — substantially higher than the 40–50% seen with unscreened embryos.

For women over 37, this argument becomes compelling. Franasiak and colleagues published landmark data in Fertility and Sterility demonstrating that aneuploidy rates rise dramatically with maternal age. In women under 35, roughly 30–40% of blastocysts are aneuploid. By age 40–42, that proportion climbs to 60–80%. By 43–44, more than 85% of blastocysts may be aneuploid. Without PGT-A, a 42-year-old patient transferring unscreened embryos is statistically likely to be transferring aneuploid embryos in most of her cycles — each resulting in either a failed implantation or an early miscarriage.

Additional evidence in favor of PGT-A:

• Lower miscarriage rate: Because aneuploid embryos are a primary cause of early pregnancy loss, selecting only euploid embryos for transfer dramatically reduces the miscarriage rate — by approximately 50% in most studies.
• Fewer failed cycles: Patients with recurrent implantation failure (multiple failed transfers with morphologically normal embryos) often have a high rate of cryptic aneuploidy. PGT-A can identify this and change the clinical strategy.
• Emotional benefit: For patients who have experienced multiple miscarriages, transferring a confirmed euploid embryo provides a measurable degree of reassurance, even if it cannot guarantee success.

The Evidence Against Routine PGT-A

For younger patients — particularly women under 35 with a good response to stimulation and a healthy embryo cohort — the picture is less clear. Several randomized controlled trials, including the STAR trial (Munne et al.), found that cumulative live birth rates were similar between PGT-A and non-PGT-A groups when all embryos were accounted for. The key insight: PGT-A improves the rate of success per transfer, but it may reduce the total number of embryos available for transfer (because some embryos scored as aneuploid might have self-corrected or been mislabeled).

There is a biological basis for this concern. Trophectoderm biopsy samples the outer layer of cells, which in some cases may not accurately represent the chromosomal composition of the inner cell mass. Cases of healthy live births from embryos that received aneuploid or high-mosaic PGT-A results have been documented, raising questions about the consequences of rigid discard policies for borderline results.

Additional arguments for caution:

• Cost: PGT-A adds $3,000 to $7,000 to an already expensive IVF cycle and is generally not covered by insurance even in states with IVF mandates.
• Lab damage risk: Trophectoderm biopsy, while performed carefully, is an invasive procedure. Any embryo manipulation carries a small but non-zero risk of damage.
• False positives: Mosaicism and technical artifacts can produce abnormal results on chromosomally normal embryos, potentially leading to the discard of a viable embryo.

The ASRM currently states that PGT-A should not be offered as a routine procedure to all IVF patients, and is most clearly beneficial for patients with advanced maternal age, recurrent pregnancy loss, and recurrent implantation failure. This nuanced position reflects the genuine state of the evidence.

Who Should Consider PGT-A?

Based on ASRM guidance and the weight of current evidence, PGT-A is most clearly beneficial for:

• Women 38 years of age or older, due to the dramatically elevated aneuploidy rate in blastocysts from older eggs
• Patients with recurrent pregnancy loss (typically defined as two or more documented miscarriages), particularly if prior losses have not been chromosomally analyzed
• Patients with recurrent implantation failure (typically three or more failed embryo transfers with morphologically normal embryos and a receptive uterine lining)
• Severe male factor infertility, where chromosomal errors in sperm may contribute to aneuploid embryos
• Couples with a prior aneuploid pregnancy, confirmed by products-of-conception testing or amniocentesis

If you are under 35, have a good ovarian response, and are entering your first IVF cycle, the data does not strongly support routine PGT-A. Your IVF success rates may be comparable whether or not you screen. That said, some patients choose PGT-A for the psychological reassurance it provides, and that is a legitimate factor in a shared decision-making conversation with your physician.

PGT-A Cost Breakdown

PGT-A is almost never covered by health insurance, even in the 20+ US states with IVF insurance mandates. The cost is charged separately from the base IVF fee and typically includes multiple components.

Pricing varies based on the number of embryos biopsied, the genetics laboratory used, and the clinic's bundling practices. Some clinics offer flat-rate pricing for up to a certain number of embryos; others charge per biopsy. Always ask for a written cost breakdown before your retrieval cycle so you're not surprised after the fact.

When evaluating cost, consider it in the context of your full treatment trajectory. If PGT-A reduces the number of transfers you need before achieving a live birth — which is genuinely possible for the right patients — it may offset some of its own cost by preventing additional frozen embryo transfer cycles, medications, and time. If you're a good-prognosis patient under 35 who would likely succeed on the first transfer anyway, the return on investment is less clear. This is a conversation worth having explicitly with your reproductive endocrinologist when choosing a fertility clinic and planning your treatment.

Mosaic Embryos — The Gray Zone

One of the most challenging developments in PGT-A has been the growing recognition of mosaic embryos — blastocysts in which some cells are chromosomally normal and others are aneuploid. Mosaicism is not a binary finding; it exists on a spectrum from low (20–40% abnormal cells) to high (40–80% abnormal cells).

For years, mosaic embryos were typically discarded alongside fully aneuploid embryos. That approach is increasingly being questioned. Registry data from cycles where mosaic embryos were transferred — particularly low-mosaic embryos — have demonstrated that live births of chromosomally normal children are possible from these embryos, likely because the normal cells in the ICM outcompete the abnormal ones as development proceeds.

However, mosaic transfers are not without risk. Outcomes are worse than with fully euploid embryos, and some clinicians counsel patients about the theoretical risk of placental abnormalities (since the trophectoderm cells biopsied had a higher proportion of aneuploid cells). Most reproductive endocrinologists recommend pursuing mosaic transfers only when no euploid embryos are available, and after thorough counseling about the limitations of the evidence.

If you receive mosaic results, ask your clinic whether they consult with their PGT-A genetics laboratory partner about which specific chromosomes are involved and what the clinical literature says about mosaicism involving those particular chromosomes — some carry a better prognosis than others.

AI-Assisted Embryo Grading — The Future of Selection

One of the most exciting developments in IVF laboratory science is the application of artificial intelligence to embryo selection. AI platforms such as iDAScore, KIDScore, and EEVA (Early Embryo Viability Assessment) analyze time-lapse videos of embryo development — captured in real-time by incubators with built-in cameras — to assign implantation probability scores based on thousands of morphokinetic data points that human embryologists cannot reliably measure.

The premise is compelling. Blastocyst grading by human embryologists is a snapshot in time. Time-lapse incubation captures the entire developmental story — how fast the embryo cleaved, whether divisions were synchronous, whether abnormal patterns (like direct cleavage from 1 cell to 3) occurred. These kinetic parameters have been associated with outcomes in retrospective studies.

A systematic review of AI-based embryo selection systems found that some AI models showed improved prediction of blastocyst development compared to standard morphology grading alone. However, the evidence for improvement in live birth rates — the outcome that matters most — remains limited and inconsistent. AI embryo grading is not yet standard of care, is not uniformly offered by clinics, and adds additional cost. That said, it represents a meaningful direction for the field, and clinics investing in time-lapse technology and AI integration are likely to accumulate the data needed to validate these tools prospectively.

If your clinic uses an AI grading system, ask what the evidence base is for the specific platform they use and whether it has been validated in their patient population.

Questions to Ask Your Embryology Team

Before and after your egg retrieval, the following questions will help you understand your embryo cohort and make informed decisions about transfer and genetic testing:

1. What grading system does your lab use? Most use the Gardner system described above, but some labs have modified or proprietary systems.
2. How do you handle mosaic PGT-A results? Do you transfer mosaics? What threshold do you use? Do you consult the genetics lab on chromosome-specific risk?
3. What is your lab's blastulation rate per retrieved egg? This is a quality metric that tells you what proportion of mature eggs your lab successfully cultures to blastocyst stage.
4. Do you use time-lapse incubation (EmbryoScope or similar)? Time-lapse incubation minimizes embryo disturbance and enables kinetic data collection.
5. What is your laboratory's accreditation status? Look for CAP (College of American Pathologists) and CLIA (Clinical Laboratory Improvement Amendments) certification. These are non-negotiable quality markers.
6. How many embryologists are on staff, and what are their credentials? A high-volume, well-staffed laboratory with senior embryologists is a significant asset.
7. What is your clinic's freeze-all versus fresh transfer policy? Understanding when your clinic recommends freezing all embryos and transferring in a subsequent cycle is relevant to the PGT-A conversation.

These questions are equally relevant whether you're entering your first IVF cycle or evaluating a new clinic after a prior unsuccessful cycle. For a broader framework on evaluating IVF programs, see our guide to choosing a fertility clinic or find an IVF clinic in our directory.

Putting It All Together

Embryo grading and PGT-A are tools — powerful ones, but tools with real limitations. A 4AA grade tells you that this embryo looked excellent on Day 5 under the microscope. A euploid result tells you that the cells sampled from its outer layer had 46 chromosomes. Neither guarantees a baby, but together they represent the best information currently available to prioritize which embryo to transfer.

The most important thing to understand is that the relationship between embryo quality and IVF outcomes is probabilistic, not deterministic. A 3BB embryo has produced successful pregnancies. A morphologically perfect 4AA euploid embryo has failed to implant. Biology is complex, and even the best embryo selection tools are predicting — not determining — outcomes.

What this means practically: if your embryo report brings difficult news — fewer blastocysts than hoped, no euploid embryos, or lower grades than you expected — take time to process it with your care team before making next-step decisions. If your IVF cycle produced one or two good-grade embryos, there is still meaningful hope. And if you are considering PGT-A, have an honest conversation with your physician about whether your specific clinical profile is one where the evidence supports the added cost and complexity.

Frequently Asked Questions

Q: What does a blastocyst grade like "4AA" actually mean? A: In the Gardner grading system, the number represents the expansion stage (4 = expanded blastocyst with thinned outer shell), the first letter is the inner cell mass quality (A = many tightly packed cells — best prognosis for fetal development), and the second letter is the trophectoderm quality (A = many cohesive cells — best for placentation and implantation). A 4AA embryo represents the highest quality designation and is associated with implantation rates of approximately 55–65% per transfer.

Q: What is PGT-A and who benefits most from it? A: PGT-A (preimplantation genetic testing for aneuploidies) screens embryos for chromosomal abnormalities before transfer using next-generation sequencing of 5–10 trophectoderm cells. It is most clearly beneficial for women 38 or older (because aneuploidy rates reach 60–80% by age 40–42), patients with recurrent pregnancy loss, and those with recurrent implantation failure. For women under 35 with good ovarian response in their first IVF cycle, the evidence does not strongly support routine PGT-A since cumulative live birth rates are similar whether or not embryos are screened.

Q: Can a mosaic embryo result in a healthy birth? A: Yes. Registry data has demonstrated live births of chromosomally normal children from low-mosaic embryos (those with 20–40% abnormal cells in the trophectoderm biopsy). Most reproductive endocrinologists recommend pursuing mosaic transfers only when no euploid embryos are available, after thorough counseling about reduced success rates compared to fully euploid embryos and the theoretical risk of placental abnormalities.

Q: How much does PGT-A add to IVF costs? A: PGT-A typically adds $3,000–$7,000 to an IVF cycle, covering embryo biopsy ($200–$400 per embryo), genetic analysis ($300–$500 per embryo), and laboratory shipping. It is almost never covered by health insurance even in states with IVF mandates. The cost should be weighed against the potential benefit of fewer failed transfers — which may offset some cost for patients who are clear candidates.

Q: Does extending embryo culture to day 5 (blastocyst) improve outcomes compared to day 3 transfer? A: A Cochrane systematic review of randomized controlled trials found that blastocyst-stage transfer improves the implantation rate per transfer compared to cleavage-stage transfer. Blastocysts that survive to day 5 have already demonstrated the capacity for sustained development. However, for patients with very few fertilized eggs, the attrition from day 3 to day 5 in the lab mirrors what would happen in the uterus — and some clinics prefer earlier transfer in those circumstances to avoid losing potentially viable embryos in culture.

For further reading on how egg quality affects the embryos you produce and what you can do to optimize your cycle, see our complete guide to egg freezing.

Medical content reviewed by Dr. Luis Arturo Ruvalcaba Castellón, MD, Reproductive Endocrinology and Infertility, Instituto Mexicano de Infertilidad / LIV Fertility Center, Guadalajara. Last reviewed April 10, 2025.

This article is for educational purposes only and does not constitute medical advice. Discuss all treatment decisions with your reproductive endocrinologist.