What is the Gardner grading system?
The Gardner system is the most widely used way to grade a blastocyst, an embryo that has grown in the lab for 5 to 7 days. It was published by David Gardner and William Schoolcraft in the late 1990s, and most US clinics use it or a close variant.
A Gardner grade has three parts, always written in the same order:
- A number from 1 to 6 for expansion: how far the fluid-filled cavity inside the embryo has grown.
- A letter, A to C, for the inner cell mass: the cluster of cells that becomes the baby.
- A second letter, A to C, for the trophectoderm: the outer layer of cells that becomes the placenta.
So a 4AA is a fully expanded blastocyst with a top-quality inner cell mass and a top-quality trophectoderm. A 3BB is less expanded, with average cells in both groups.
Grading is visual. An embryologist looks at the embryo under a microscope, or on a time-lapse image, and scores what can be seen. It is a skilled judgment, not a lab test, and two embryologists can grade the same embryo slightly differently. Grading tells you about structure and development. It does not tell you about chromosomes. That is what PGT-A is for.
| Component | Grade | What the embryologist sees | What it suggests |
|---|---|---|---|
| Expansion | 1 | Early blastocyst; cavity less than half the embryo | Still developing; usually graded again later |
| Expansion | 2 | Cavity half or more of the embryo | Still developing |
| Expansion | 3 | Full blastocyst; cavity fills the embryo | Ready to assess both cell groups |
| Expansion | 4 | Expanded; cavity larger than the original embryo, shell thinning | Fully expanded |
| Expansion | 5 | Hatching; embryo starting to push out of its shell | Fully expanded and hatching |
| Expansion | 6 | Hatched; embryo fully out of its shell | Fully hatched |
| Inner cell mass | A | Many cells, tightly packed | Top quality |
| Inner cell mass | B | Several cells, loosely grouped | Good to average |
| Inner cell mass | C | Very few cells | Below average |
| Trophectoderm | A | Many cells forming a cohesive layer | Top quality |
| Trophectoderm | B | Few cells forming a loose layer | Good to average |
| Trophectoderm | C | Very few, large cells | Below average |
| Example | 4AA, 5AA | Fully expanded or hatching, top-quality cells | Top grades; treated similarly |
| Example | 4AB, 4BA, 3AA | Expanded, one strong and one good cell group | Good grades |
| Example | 3BB, 4BB | Expanded, average cells | Average grades; many lead to healthy births |
| Example | 4BC, 3CC | Expanded, one or both cell groups below average | Lower grades; can still lead to healthy births |
Clinics vary in how they use the scale. Some add plus or minus signs, some report only the letters, and some use a numeric or descriptive version. If your report uses a different format, ask your embryologist to translate it to the Gardner scale. Common terms on embryology reports are also defined in the surrogacy glossary.
What does the expansion number mean?
The number describes how far the blastocyst has developed. As the embryo grows, it fills with fluid, the cavity enlarges, and the shell around it, the zona pellucida, stretches and thins.
Grades 1 and 2 are early blastocysts. The two cell groups are often not yet clear enough to grade, so many clinics wait and look again. Grade 3 is a full blastocyst. Grade 4 is fully expanded. Grades 5 and 6 are hatching and hatched, which is what an embryo does before it can attach to the uterine wall.
For practical purposes, 4, 5 and 6 are all well-developed embryos. A 5AA is not meaningfully better than a 4AA. The number mostly reflects when the embryologist happened to look.
What do the inner cell mass and trophectoderm letters mean?
The first letter grades the inner cell mass, the group of cells that will form the fetus. A means many cells, tightly packed. B means several cells, loosely grouped. C means very few cells.
The second letter grades the trophectoderm, the single layer of cells around the outside that will form the placenta. A means many cells forming a cohesive layer. B means fewer cells in a looser layer. C means very few, large cells.
The trophectoderm also matters because it is the part of the embryo that attaches to the uterus. In PGT-A, it is also where the lab takes its biopsy, so the inner cell mass is not touched.
Letters describe appearance, not health. A B is not a failing grade. Many healthy babies come from BB and BC embryos. That includes babies born through PLS journeys.
How does grade relate to implantation and live birth?
On average, higher-grade blastocysts implant and lead to live birth more often than lower-grade ones. Studies generally find the trophectoderm and the expansion stage are linked to outcomes, which makes sense, since the trophectoderm is the part that attaches to the uterus.
Two cautions keep grade in proportion.
First, grade is a probability, not a verdict. A lower grade lowers the odds. It does not rule out a healthy pregnancy, and a top grade does not guarantee one.
Second, grade does not see chromosomes. A beautiful 4AA embryo can have the wrong number of chromosomes and fail to implant or miscarry. A modest 4BC can be euploid and become a healthy baby. When embryos have been tested, euploid status matters more than any single letter.
PLS does not publish implantation percentages by grade, because they vary with egg provider age, clinic, lab and whether the embryo was tested. Your clinic can show you its own data.
Day 5, day 6 and day 7 blastocysts: does the day matter?
Embryos reach the blastocyst stage at different speeds. Some are ready to biopsy or freeze on day 5, others on day 6, and a smaller number on day 7.
On average, embryos that reach blastocyst on day 5 do somewhat better than slower ones. In many studies, the difference narrows once the embryos have been tested and found euploid. A euploid day 6 blastocyst is a strong embryo. Day 7 blastocysts are less common, and some clinics do not keep them; others do, especially when a family has few embryos.
The day is usually written on the embryology report next to the grade, for example "day 6, 4AB." Ask about both together.
The day also matters for the surrogate's transfer cycle. Her progesterone start is timed to the embryo's age, so the clinic needs to know whether it is transferring a day 5, 6 or 7 embryo. The frozen embryo transfer protocol page explains that timing.
How does grading work together with PGT-A?
Grading and PGT-A answer different questions.
- Grading asks: how well is this embryo developing, judged by eye?
- PGT-A asks: does this embryo have the expected number of chromosomes?
In a typical surrogacy cycle, the lab grades each blastocyst, biopsies a few trophectoderm cells from those good enough to test, and freezes them. The genetics lab reports each tested embryo as euploid, aneuploid or mosaic, usually within 1 to 2 weeks.
The result is a short list: euploid embryos, each with a grade and a day. That list is what your clinic uses to choose which embryo to transfer. Most surrogacy clinics and agencies recommend transferring euploid embryos, because doing so lowers the risk of miscarriage and avoids asking a surrogate to go through a transfer that is unlikely to succeed.
Which embryo will the clinic transfer first to the surrogate?
Usually the one with the best combination of three factors:
- Euploid status, if the embryos were tested.
- Grade, with expansion and trophectoderm weighed most by many labs.
- Day, with a day 5 embryo often chosen before a day 6 of similar grade.
Clinics vary in how they rank these, and some also weigh time-lapse observations. You make the choice with your reproductive endocrinologist. The clinic decides what is medically appropriate.
In nearly all PLS journeys, one embryo is transferred, following ASRM guidance on single embryo transfer. The IVF for surrogacy page walks through the full sequence from retrieval to transfer.
This page gives general information about embryo grading. It is not medical advice. Your clinic's embryologist and reproductive endocrinologist interpret your embryos.
Sources
- Gardner DK, Schoolcraft WB, In vitro culture of human blastocysts, in Towards Reproductive Certainty (1999), the original description of the grading system.
- ASRM (American Society for Reproductive Medicine), guidance on the number of embryos to transfer and on PGT-A. https://www.asrm.org/
- SART (Society for Assisted Reproductive Technology), clinic outcome reports. https://www.sart.org/