Donor Nexus Team
September 16, 2025
what is embryo grading?
An embryo grade is a visual score an embryologist records when an embryo is frozen. In the Gardner system, a number from 1 to 6 describes how expanded the blastocyst is, and two letters from A to C rate its inner cell mass and trophectoderm.1 Higher grades are linked with higher pregnancy rates, yet lower-grade embryos still result in pregnancies.
Grade is one factor among several. Your physician weighs it alongside the day the embryo was frozen, whether it has had chromosome testing, and your own health history.
What is embryo grading?
IVF embryo grading is the way embryologists describe what an embryo looks like under the microscope at a specific point in its development. On a donor embryo profile, the grade usually sits next to the day the embryo was frozen and its genetic testing status, so it is often one of the first details people notice. You will see a grade listed on profiles in the Donor Embryo Program, and knowing what the numbers and letters describe can make those profiles much easier to compare.
In this guide, you can use an embryo grading chart to see how the Gardner blastocyst scale works, learn how to read the HRC Fertility letter scale used on Donor Nexus profiles, and review what published research says about embryo grading and success rates, including how the day an embryo was frozen relates to outcomes. You will also see what a grade can't show you and which questions to bring to your physician.
What does an embryo grading chart show?
An embryo grading chart shows what each part of a grade rates and which end of each scale is highest, so you can decode a grade like 4AA or XBBB at a glance.1,2 IVF embryo grading for blastocysts, the embryos that have developed for five to seven days, usually follows Gardner's format of one number and two letters, while day 3 embryos are graded on their cells and fragmentation instead.12
| Part of the grade | What it rates | Scale | Top of the scale | Bottom of the scale |
|---|---|---|---|---|
| Number (the 4 in 4AA) | Blastocyst expansion | 1 to 6 | 6: hatched blastocyst that has fully left the zona | 1: early blastocyst with a small cavity |
| First letter (the first A in 4AA) | Inner cell mass, which becomes the fetus | A to C | A: tightly packed, many cells | C: very few cells |
| Second letter (the second A in 4AA) | Trophectoderm, which forms the placenta | A to C | A: many cells forming a cohesive layer | C: very few large cells |
| HRC Fertility letter grade (used on Donor Nexus profiles) | Blastocyst stage and letter grades combined | A to C- | A: fully hatching or hatching blastocyst graded BB or better | C-: compacting embryo |
| Day 3 grade (cleavage-stage embryos) | Fragmentation, cell size, and multinucleation | Grade 1 to 3 | Grade 1 (good): less than 10% fragmentation and even, stage-appropriate cell size | Grade 3 (poor): more than 25% fragmentation, uneven cell size, and multinucleation |
Embryo grading chart summarizing the Gardner blastocyst scale, HRC Fertility's letter scale, and day 3 grading. Sources: Gardner et al., 2000; HRC Fertility via Donor Nexus; Istanbul consensus, 2011.1,2,12
The rest of this guide focuses on blastocyst grades, because those are the grades described on the Donor Nexus profiles covered here. Day 3 grading is more likely to appear in records from your own earlier IVF cycles, where a day 3 embryo is expected to have about eight cells.12
How does the Gardner blastocyst grading system work?
The Gardner system gives a blastocyst one number and two letters: the number describes how far the embryo has expanded, and the letters rate its two main groups of cells.1 David Gardner and colleagues described the scale in a 2000 study that linked blastocyst scores with implantation and pregnancy outcomes, and the grading studies cited later in this article report embryo quality in the same format.1
A blastocyst is an embryo that has formed a fluid-filled cavity, called the blastocoel, surrounded by an outer shell called the zona. Inside it are two cell groups that you will see graded on a profile: the inner cell mass, which becomes the fetus, and the trophectoderm, the outer layer of cells that forms the placenta.
What do the expansion numbers 1 to 6 mean?
The number tells you the blastocyst's expansion stage, running from an early blastocyst with a small cavity at 1 to a hatched blastocyst that has fully left the zona at 6.1
| Expansion grade | Stage | Definition |
|---|---|---|
| 1 | Early blastocyst | The blastocoel fills less than half of the embryo's volume. |
| 2 | Blastocyst | The blastocoel fills half or more of the embryo's volume. |
| 3 | Full blastocyst | The blastocoel completely fills the embryo. |
| 4 | Expanded blastocyst | The blastocoel volume is larger than that of the early embryo, and the zona is thinning. |
| 5 | Hatching blastocyst | The trophectoderm is starting to herniate through the zona. |
| 6 | Hatched blastocyst | The blastocyst has completely escaped from the zona. |
Embryo grading chart for Gardner blastocyst expansion grades, adapted from Gardner et al., 2000.1
What do the inner cell mass and trophectoderm letters mean?
The first letter grades the inner cell mass and the second letter grades the trophectoderm, and in Gardner's scale both letter grades are assigned to blastocysts at expansion stage 3 or higher.1 An A describes the most cells and the most organized structure, while a C describes very few cells.
| Letter grade | Inner cell mass (first letter) | Trophectoderm (second letter) |
|---|---|---|
| A | Tightly packed, many cells | Many cells forming a cohesive epithelium (cell layer) |
| B | Loosely grouped, several cells | Few cells forming a loose epithelium |
| C | Very few cells | Very few large cells |
Embryo grading chart for Gardner inner cell mass and trophectoderm grades, adapted from Gardner et al., 2000.1
How do you read a grade like 4AA?
You read a Gardner grade from left to right, so in 4AA the 4 means an expanded blastocyst, the first A means a tightly packed inner cell mass with many cells, and the second A means a trophectoderm with many cells forming a cohesive layer.1 A 3BB, by comparison, is a full blastocyst with a loosely grouped inner cell mass and a looser trophectoderm. Neither grade tells you anything about the embryo's chromosomes, which is why grading and genetic testing answer different questions.
How to read a blastocyst grade such as 4AA, plus the HRC Fertility letter scale. Sources: Gardner et al., 2000; HRC Fertility grading via Donor Nexus.1,2
How do you read a grade on a Donor Nexus embryo profile?
On Donor Nexus profiles, you may see grades written in HRC Fertility's shorthand, which names the blastocyst stage with letters and then lists the inner cell mass and trophectoderm grades.2 Donor Nexus works with HRC Fertility, where Donor Embryo Program transfers take place, and the grading classifications on profiles follow HRC Fertility's scale.
What does "Day 6, Grade: XBBB" mean?
On profiles you may see a card that reads "Day 6, Grade: XBBB," which describes an embryo frozen on day 6 of development that was recorded as an expanding blastocyst (XB), with an inner cell mass grade of B and a trophectoderm grade of B.2 Under HRC Fertility's letter scale, an expanding blastocyst graded BB or AA corresponds to an A-/B+.2
Abbreviations are not identical from one lab to the next, and HRC Fertility's stage names don't line up number for number with Gardner's 1 to 6 scale. If a profile uses a code you don't recognize, ask the Donor Nexus team or your physician to confirm how it was recorded before you compare it with other profiles.
How does the HRC Fertility letter scale work?
HRC Fertility's scale combines an embryo's stage and its two letter grades into a single letter grade, from A for a fully hatching or hatching blastocyst down to C- for an embryo that is still compacting.2
| Embryo stage and grades | HRC Fertility letter grade |
|---|---|
| Fully hatching blastocyst (FHB) or hatching blastocyst (HB), graded BB or better | A |
| Expanding blastocyst (XB), graded BB or AA | A-/B+ |
| Blastocyst (B), graded BB or AA | B/B- |
| Early blastocyst (EB), graded AA or BB | C+/C |
| Compacting embryo (Comp) | C- |
Embryo grading chart for HRC Fertility's letter scale, as published by Donor Nexus. Your physician can explain what a letter grade means for your treatment plan.2
Does it matter whether an embryo was frozen on day 5, 6, or 7?
The day an embryo was frozen may matter, but the evidence is mixed: some research links day 5 blastocysts with higher live birth rates, while studies limited to chromosome-tested embryos show smaller or less consistent differences between day 5 and day 6.
A 2019 meta-analysis of 29 observational studies found live birth rates were higher after day 5 blastocyst transfers than after day 6 transfers overall, with a relative risk of 1.50, and the difference held in both fresh and vitrified transfers.3 Among euploid (chromosomally normal) embryos, a 2019 study reported sustained implantation rates of 68.9% for day 5, 66.8% for day 6, and 52.6% for day 7 blastocysts, and none of those differences was statistically significant, although the day 7 group was small.4 A 2022 study of single euploid frozen embryo transfers in 2,966 patients found live birth rates of 68.5%, 55.2%, and 36.0% for day 5, 6, and 7 blastocysts, with day 7 significantly lower than both earlier days.5
| Study | Embryos studied | Day 5 | Day 6 | Day 7 |
|---|---|---|---|---|
| Bourdon et al., 2019 (meta-analysis) | 29 observational studies of fresh and frozen blastocyst transfers | Higher live birth rate (relative risk 1.50 vs day 6) | Comparison group | Not compared |
| Tiegs et al., 2019 | Euploid frozen blastocyst transfers | 68.9% sustained implantation | 66.8% sustained implantation | 52.6% sustained implantation (differences not significant) |
| Lane et al., 2022 | Single euploid frozen transfers, 2,966 patients | 68.5% live birth | 55.2% live birth | 36.0% live birth |
The studies measured different outcomes in different patient groups, so their figures are not directly comparable.3,4,5
If a profile you like lists a day 6 embryo, these studies show that day 6 euploid embryos resulted in pregnancies and live births at substantial rates. Day 7 embryos had lower live birth rates in the larger of the two euploid studies, so ask your physician how the day frozen fits together with the grade and testing status before you rule a profile in or out.
How does embryo grading relate to success rates?
Embryo grading and success rates are related, since studies of chromosome-tested embryos found higher-grade blastocysts had higher ongoing pregnancy rates, yet lower-grade embryos still led to pregnancies in those same studies.6,7
What does research show about embryo grade and success rates?
A 2017 study of frozen euploid blastocyst transfers at an academic medical center grouped embryos by their grade before freezing and found ongoing pregnancy rates of 84.2% for excellent blastocysts (3AA or higher) and 35.8% for poor blastocysts.6 A 2019 study of 2,236 single frozen euploid blastocyst transfers found an ongoing pregnancy or live birth rate of 55.6% when the inner cell mass was graded A, compared with 32.3% when it was graded C, and the authors concluded that inner cell mass morphology was the strongest predictor of sustained implantation among euploid embryos.7 Both studies were retrospective, which means they describe past patterns and can't isolate grade from every other factor.
| Grade component | Grade A | Grade B | Grade C |
|---|---|---|---|
| Inner cell mass (first letter) | 55.6% | 43.1% | 32.3% |
| Trophectoderm (second letter) | 55.7% | 52.8% | 40.0% |
Embryo grade and success rates: ongoing pregnancy or live birth rate by grade across 2,236 single frozen euploid blastocyst transfers, from Nazem et al., 2019. The rates describe groups of embryos, not the chance for any single embryo.7
For you, these findings suggest a higher grade can tilt the odds, especially a stronger inner cell mass grade, while a B or C grade doesn't rule out a pregnancy. Your physician may weigh grade together with testing status, the embryo donors' ages when the embryos were frozen, and the results of your own pretesting.
How is embryo grading different from PGT-A testing?
Embryo grading describes how an embryo looks, while preimplantation genetic testing for aneuploidy (PGT-A) checks whether it has the expected number of chromosomes by analyzing a small biopsy of cells. A high grade does not confirm normal chromosomes, and the poor-grade euploid embryos in the research above show that a lower grade does not mean an embryo is abnormal.6
Professional guidance on PGT-A is cautious. The American Society for Reproductive Medicine (ASRM) states that the value of PGT-A as a routine screening test for all IVF patients has not been demonstrated,8 and the American College of Obstetricians and Gynecologists (ACOG) notes that false-positive and false-negative results are possible.9 Some profiles list PGT-A results, and you can request PGT-A on untested embryos, so read more about PGT-A testing at Donor Nexus and discuss with your physician whether it fits your plan.2
Do freezing and warming affect a graded embryo?
The grade on a profile describes the embryo at the time it was frozen, and current vitrification methods allow most embryos to survive warming. ASRM and the Society of Reproductive Biologists and Technologists describe embryo survival rates after vitrification as approaching 100%.10 If an embryo does not survive warming, Donor Nexus can thaw another embryo from the same profile or from a backup profile, and if a backup profile doesn't meet your criteria, a refund or credit is available.
What can't an embryo grade tell you?
An embryo grade can't tell you whether the embryo has the expected number of chromosomes, how your uterus will respond to the transfer, or how one lab's scale compares with another's. Chromosome status requires testing such as PGT-A, and your uterine environment is evaluated separately during pretesting, usually with a uterine cavity evaluation that no embryo score reflects. Grading is also a visual judgment made by an embryologist, and labs use different scales and abbreviations, as the difference between the Gardner and HRC Fertility formats shows. Because choosing between an A, B, or C involves some judgment, an embryo that one embryologist grades A may be graded B by another, even at the same clinic.11
What should you ask your physician about a profile's grade?
Bring each profile's day frozen, grade, and testing status to your physician and ask how those details apply to your own history and treatment plan. These questions can help you structure that conversation.
- What stage and grade did the embryologist record, and on which day was the embryo frozen?
- How does this grade on HRC Fertility's letter scale compare with the Gardner grades used in the research?
- Has this embryo had PGT-A, and would you suggest requesting testing if it hasn't?
- How do the embryo donors' ages when the embryos were frozen affect your view of this profile?
- Are outcomes known for other embryos from the same batch, and do they change your recommendation?
- If the profile includes more than one embryo, which one would you transfer first, and why?
- Based on my pretesting results, how much weight should grade carry compared with other factors?
Your physician's answers can help you compare profiles side by side. For a walk-through of everything else on a profile, read our guide on how to choose a donor embryo, and for national and program outcome data, see donor embryo success rates.
Frequently asked questions about embryo grading
What is a good embryo grade?
There is no single cutoff. In the research cited in this guide, blastocysts graded 3AA or higher were grouped as excellent, and embryos with an A or B inner cell mass had higher ongoing pregnancy or live birth rates than those graded C. On HRC Fertility's letter scale, A and A-/B+ are the top grades, and your physician can explain how a specific grade fits your treatment plan.
Is a 4AA embryo more likely to result in pregnancy than a 3BB embryo?
In published studies, embryos with higher Gardner grades, particularly a higher inner cell mass grade, have had higher pregnancy rates on average. Those results describe groups of embryos, not any single embryo, so a 3BB embryo can still result in a pregnancy, and your physician can put the comparison in context for your situation.
Can a C-grade embryo still lead to a pregnancy?
Yes, it can. In a 2019 study of chromosome-tested embryos, those with a C-grade inner cell mass reached ongoing pregnancy or live birth in about a third of transfers, a lower rate than A-grade embryos but far from zero. On HRC Fertility's scale, C grades describe early blastocysts and compacting embryos.
Should I pick a day 5 embryo over a day 6 embryo?
Not necessarily. Some research links day 5 blastocysts with higher live birth rates, while a study of chromosome-tested embryos found similar implantation rates for day 5 and day 6. Your physician may weigh the day frozen together with the grade, any PGT-A results, and the embryo donors' ages at freezing.
Does a high grade mean an embryo is chromosomally normal?
No. A grade reflects how an embryo looks under a microscope, and chromosome count can't be seen that way. PGT-A tests a biopsy sample for chromosome number, although false-positive and false-negative results are possible, and ACOG recommends offering prenatal screening or diagnostic testing to patients who conceive after PGT-A.
Who assigns the grade listed on an embryo profile?
An embryologist assigns the grade when the embryo is frozen, based on how it looks at that moment. Because donor embryos were created during the donating family's own IVF treatment, the grade reflects that lab's assessment. If a profile's abbreviations are unclear, the Donor Nexus team can help you confirm what was recorded.
When a profile's grade and day frozen look promising, your next step is to browse profiles and bring a shortlist to your physician. You can register for free to view full profiles in the embryo bank, learn more about embryo donation at Donor Nexus, or contact the team at info@donornexus.com or +1 (949) 433-5635, Monday to Friday, 8 a.m. to 4 p.m. Pacific, with questions about any grade you see.
Sources
- Gardner DK, Lane M, Stevens J, Schlenker T, Schoolcraft WB. Blastocyst score affects implantation and pregnancy outcome: towards a single blastocyst transfer. Fertility and Sterility. 2000;73(6):1155-1158. doi:10.1016/S0015-0282(00)00518-5.
- Donor Nexus. Embryo donation process: embryo information and screening. Accessed September 2026. donornexus.com/embryo-donation-process.
- Bourdon M, Pocate-Cheriet K, Finet de Bantel A, Grzegorczyk-Martin V, Amar Hoffet A, Arbo E, Poulain M, Santulli P. Day 5 versus Day 6 blastocyst transfers: a systematic review and meta-analysis of clinical outcomes. Human Reproduction. 2019;34(10):1948-1964. doi:10.1093/humrep/dez163.
- Tiegs AW, Sun L, Patounakis G, Scott RT. Worth the wait? Day 7 blastocysts have lower euploidy rates but similar sustained implantation rates as Day 5 and Day 6 blastocysts. Human Reproduction. 2019;34(9):1632-1639. doi:10.1093/humrep/dez138.
- Lane SL, Reed L, Schoolcraft WB, Katz-Jaffe MG. Euploid day 7 blastocysts of infertility patients with only slow embryo development have reduced implantation potential. Reproductive BioMedicine Online. 2022;44(5):858-865. doi:10.1016/j.rbmo.2021.08.027.
- Irani M, Reichman D, Robles A, Melnick A, Davis O, Zaninovic N, Xu K, Rosenwaks Z. Morphologic grading of euploid blastocysts influences implantation and ongoing pregnancy rates. Fertility and Sterility. 2017;107(3):664-670. doi:10.1016/j.fertnstert.2016.11.012.
- Nazem TG, Sekhon L, Lee JA, Overbey J, Pan S, Duke M, Briton-Jones C, Whitehouse M, Copperman AB, Stein DE. The correlation between morphology and implantation of euploid human blastocysts. Reproductive BioMedicine Online. 2019;38(2):169-176. doi:10.1016/j.rbmo.2018.10.007.
- Practice Committees of the American Society for Reproductive Medicine and the Society for Assisted Reproductive Technology. The use of preimplantation genetic testing for aneuploidy: a committee opinion. Fertility and Sterility. 2024;122(3):421-434. doi:10.1016/j.fertnstert.2024.04.013.
- American College of Obstetricians and Gynecologists. Preimplantation genetic testing: ACOG Committee Opinion No. 799. Obstetrics & Gynecology. 2020;135(3):e133-e137. doi:10.1097/AOG.0000000000003715.
- Practice Committees of the American Society for Reproductive Medicine and the Society of Reproductive Biologists and Technologists. A review of best practices of rapid-cooling vitrification for oocytes and embryos: a committee opinion. Fertility and Sterility. 2021;115(2):305-310. doi:10.1016/j.fertnstert.2020.11.017.
- Lauber S. IVF embryo grades explained (3AA, 5CC, 4BB + success rates). Remembryo. Updated September 4, 2026. Accessed September 2026. remembryo.com/embryo-grading.
- Alpha Scientists in Reproductive Medicine and ESHRE Special Interest Group of Embryology. The Istanbul consensus workshop on embryo assessment: proceedings of an expert meeting. Human Reproduction. 2011;26(6):1270-1283. doi:10.1093/humrep/der037.
Related reading
Medical disclaimer
This article shares general information about embryo grading and is not medical advice. Your fertility doctor knows your history and treatment plan, so their guidance comes first. National figures and Donor Nexus outcome data describe past results and do not predict the outcome of any individual cycle.
Last updated September 2026. Figures cited here are drawn from the sources listed above.
Donor Nexus Team
The Donor Nexus Team is the group of case managers, donor coordinators, and egg bank staff who run the agency's Fresh Donor Matching, Frozen Egg Bank, and Embryo Donation programs day to day. Since 2012 they ha…
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