How are blastocysts graded?
Most embryos used in surrogacy are frozen at the blastocyst stage, on day 5 or day 6 after fertilization. By then the embryo has a fluid-filled cavity and two distinct groups of cells. The embryologist grades it by looking at it under a microscope, on three features:
- Expansion, graded 1 to 6. How far the blastocyst has grown and how large its cavity is. A 1 is an early blastocyst. A 3 is a full blastocyst. A 4 is expanded. A 5 is hatching out of its shell. A 6 has fully hatched. Most embryos frozen for transfer are 3 or higher.
- Inner cell mass (ICM), graded A to C. The cluster of cells that becomes the baby. A means many tightly packed cells. B means fewer or more loosely grouped cells. C means very few cells.
- Trophectoderm (TE), graded A to C. The outer layer of cells that becomes the placenta. A means many cells forming a cohesive layer. B means fewer cells or a looser layer. C means very few cells.
The three are written together: a number and two letters. A 4AA is an expanded blastocyst with an excellent inner cell mass and an excellent trophectoderm. A 5BB is a hatching blastocyst with good but not top grades on both cell groups.
This scale is widely used, but clinic grading systems vary. Some add their own categories, some grade on day 5 and day 6 separately, and some use a numeric score instead of letters. Ask your clinic which system it uses and what its own grades mean.
| Grade component | What it describes | General implantation expectation | Transfer priority |
|---|---|---|---|
| Expansion 4 to 6 | Expanded, hatching or hatched blastocyst | Generally favorable | Higher |
| Expansion 3 | Full blastocyst | Generally favorable, slightly less mature | Higher to middle |
| Expansion 1 to 2 | Early blastocyst | Lower; often watched longer before freezing | Lower |
| ICM A | Many tightly packed cells | More favorable | Higher |
| ICM B | Fewer or looser cells | Favorable | Middle |
| ICM C | Very few cells | Less favorable | Lower |
| TE A | Many cells in a cohesive layer | More favorable | Higher |
| TE B | Fewer cells or looser layer | Favorable | Middle |
| TE C | Very few cells | Less favorable | Lower |
These expectations are qualitative. Grade-specific implantation percentages vary by clinic, by lab and by the age of the egg source, and are not stated here.
What does a grade like 4AA or 3BC mean for success?
A grade is a prediction, not a verdict. Higher grades generally mean a higher chance that the embryo will implant and lead to a pregnancy. Lower grades mean a lower chance per transfer. But lower-graded embryos still produce healthy babies.
A 4AA is the kind of embryo clinics prefer to transfer first: well expanded, with strong cells in both groups. A 3BC is a full blastocyst with a good inner cell mass and a weaker trophectoderm. Its chance per transfer is lower than a 4AA's, but it remains a viable embryo, and many clinics would transfer it if it is the best one available or if it is chromosomally normal.
Grades describe appearance on one day. They do not tell you the embryo's chromosomes. Two embryos with the same grade can have very different outcomes because one is chromosomally normal and the other is not.
Grade versus PGT-A: what each tells you
Grading and PGT-A answer different questions.
- Grading tells you what the embryo looks like. Every clinic does it, at no added step. It predicts the chance of implantation.
- PGT-A, preimplantation genetic testing for aneuploidy, tells you whether the embryo has the normal number of chromosomes. It requires a biopsy of a few cells from the trophectoderm and costs extra. It does not tell you the embryo's appearance.
A beautiful embryo can be aneuploid, meaning it has an abnormal number of chromosomes, and an aneuploid embryo usually does not implant or ends in miscarriage. A modest-looking embryo can be euploid, meaning chromosomally normal, and lead to a healthy birth.
Both are useful. Grading is automatic. PGT-A is optional and adds chromosomal information that grading cannot. In surrogacy, where each transfer involves another person's body and months of time, many parents choose both. See PGT-A testing before surrogacy.
Which embryo is transferred first, and who decides?
Your fertility clinic's physician decides, with you. The clinic selects the embryo with the best combination of grade and, when tested, a euploid result.
The usual order, in plain terms:
- Euploid embryos before untested ones, where testing was done.
- Among euploid embryos, higher grades first.
- Where grades are similar, many clinics prefer day-5 embryos over day-6 embryos.
The number of embryos is not decided on transfer day. At Private Label Surrogacy (PLS) it is agreed at matching and written into the gestational carrier agreement. ASRM recommends single embryo transfer for gestational carriers, and usually one embryo is transferred. See single vs double embryo transfer.
How does embryo quality shape the number of transfers you should plan for?
Embryo quality, not the carrier, is the main driver of success per transfer. A PLS surrogate has already carried at least one healthy full-term pregnancy. Mark Silverberg, DDS, MD, has read her prenatal and delivery records, and your clinic has evaluated her uterus. A screened carrier with a proven uterus removes uterine factors. The embryo's own quality still sets the ceiling.
So plan your journey around your embryos:
- Several euploid, high-grade embryos. You can reasonably plan for success within one or two transfers, while budgeting for another.
- One or two euploid embryos. Plan for the possibility that you need to create more.
- Untested or lower-graded embryos. Plan for more transfers, or talk with your clinic about creating and testing more embryos before you match.
Many clinics suggest at least two good-quality embryos, ideally euploid, before committing to a surrogacy program. Ask your reproductive endocrinologist what they recommend for your embryos.
Each additional transfer adds time, usually 2 to 3 months for a new cycle, and costs. For what happens after a negative test, see failed transfer and miscarriage.
Day 5 versus day 6 embryos
Some embryos reach the blastocyst stage on day 5. Others take until day 6. A small number reach it on day 7, and some clinics freeze those too.
Many studies and clinics report somewhat better outcomes for day-5 blastocysts than for day-6 blastocysts of the same grade. The difference is smaller for euploid embryos, and a euploid day-6 blastocyst remains a good embryo to transfer.
Ask your clinic which day each of your embryos was frozen, and how the clinic weighs day 5 against day 6 in choosing the order of transfer.
What to ask your clinic before you sign with an agency
Before you sign with PLS or any agency, ask your clinic for:
- The grade of every frozen embryo, in writing.
- The grading system the clinic uses, and what its top and bottom grades mean.
- The day each embryo was frozen: day 5, 6 or 7.
- PGT-A status: euploid, aneuploid, mosaic or untested, for each embryo.
- The clinic's recommendation on how many transfers to plan for with these embryos.
- Whether the clinic transfers to gestational carriers, and its own requirements for a carrier.
- Shipping, if your embryos will move to another clinic for the transfer.
Bring these answers to your first consultation. They shape your timeline, your budget, and which surrogates will be interested in your profile, since many surrogates ask about embryo status in matching. For how PLS reports its own results, see surrogacy success rates.
Sources
- ASRM (American Society for Reproductive Medicine), committee opinions on embryo transfer and on gestational carriers. https://www.asrm.org
- SART (Society for Assisted Reproductive Technology), national IVF outcome reporting. https://www.sart.org