Before an IVF embryo is transferred, embryologists assess how it has developed and what it looks like under the microscope. They use that information to rank embryos, not to guarantee which one will implant or lead to a live birth. Some patients also consider PGT-A, a separate test of chromosome number; it is optional and cannot guarantee a baby.
What embryologists assess before transfer
Embryo assessment generally combines developmental timing—how far an embryo has progressed and how quickly—with morphology, its visible structure and cell organization. The precise observations and grading conventions can vary between clinics and laboratories. The updated ESHRE/ALPHA Istanbul Consensus, published in 2025, sets out recommended static and dynamic morphology criteria for assessment and ranking; ASRM describes overall morphology grading as subjective.
Morphology helps a clinic compare the embryos available in a cycle. It does not reveal every chromosomal or developmental issue, and a visually strong embryo is not thereby proven to have a normal chromosome number.
Cleavage-stage embryos and blastocysts
Clinics may assess embryos at the cleavage stage, commonly on day 2 or 3, or continue culturing them to the blastocyst stage, commonly day 5 or 6. Continuing culture provides more information about which embryos keep developing, but some embryos do not reach the blastocyst stage. For someone with few embryos, that can mean there is no embryo available to transfer at that stage. As the UK Human Fertilisation and Embryology Authority (HFEA) notes, it is not possible to know whether a particular embryo that did not reach blastocyst would have continued to a successful pregnancy if transferred earlier.
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| Option | What the clinic can assess | Key consideration |
|---|---|---|
| Cleavage-stage assessment or transfer, commonly day 2 or 3 | Cell number, rate of division, whether divisions look even, and whether cell fragments are present | Assessment is made earlier in development; it cannot show whether that embryo would later have reached the blastocyst stage. |
| Continue culture to blastocyst, commonly day 5 or 6 | Further development and, if a blastocyst forms, its expansion and the appearance of its inner cell mass and trophectoderm | It gives additional information for ranking, but some embryos do not reach this stage and may not be available for transfer. |
There is no universally right choice between these approaches. The number of embryos, the clinic’s laboratory practice, and the patient’s circumstances inform the discussion.
How to read a blastocyst grade
A commonly used system describes a blastocyst with a number and two letters. The number refers to its expansion and hatching stage; the letters describe the inner cell mass (ICM) and trophectoderm (TE). The ICM contributes to the fetus, while the TE contributes to supporting tissues. Clinics can use different grading conventions, so ask your clinic to interpret the grade using its own system.
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| Stage number | What the number describes |
|---|---|
| 1 | Early blastocyst, with a small cavity. |
| 2 | The grading resource identifies this within the numerical expansion scale; it does not provide a separate plain-language description for this stage. |
| 3 | Full blastocyst: the cavity fills the embryo. |
| 4 | Expanded blastocyst: the cavity is larger and the outer shell is thinning. |
| 5 | Hatching blastocyst: it is beginning to emerge from its shell. |
| 6 | Hatched blastocyst: it has escaped the shell. |
For stages 3–6, the ICM description reflects how many cells it has and how tightly grouped they are. The TE description reflects the number of cells and whether they form a cohesive layer. The letters are morphology descriptions, not genetic test results. A grade can help prioritize embryos, but no grade or cutoff is established as a guarantee of live birth.
What PGT-A adds—and what it cannot tell you
Preimplantation genetic testing for aneuploidy (PGT-A) is an additional chromosome-number assessment, not part of ordinary visual grading. In the commonly described approach, a few cells are biopsied from a blastocyst and tested; the result is used to represent the embryo as a whole. The result may be reported as euploid, aneuploid, mosaic, or no result.
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- Mosaic: the tested sample contains cells with different chromosome findings. The proportion reported and how it is interpreted matter; clinics may differ in their reporting and transfer policies.
- No result: the test has not produced a result that can be used for classification. Ask the fertility team what the clinic’s options and policies are.
PGT-A does not guarantee implantation, pregnancy, or a live birth. It can also reduce the number of embryos available for transfer: a result may be inaccurate, or biopsy and testing may result in a viable embryo being unavailable. For an uncertain or mosaic result, discuss its interpretation and any transfer decision with the fertility team and, where appropriate, a genetic counselor.
Is PGT-A routine for every IVF patient?
No. In its 2024 committee opinion, the American Society for Reproductive Medicine (ASRM) says routine PGT-A screening for all IVF patients has not been shown to provide value and that routine blastocyst biopsy with PGT-A in all infertile patients cannot currently be recommended. The HFEA’s patient guidance says randomized-trial evidence has not shown that blastocyst-stage PGT-A improves the chance of having a baby for most IVF patients. These positions do not rule out an individualized discussion; they do mean PGT-A should not be presented as a universal best practice.
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Whether to consider it depends on the patient’s circumstances and priorities, including age, history, and the number of embryos. Ask the clinic what benefit it expects in your situation, what the possible results would mean, and what alternatives are available. Evidence about PGT-A varies by patient group and outcome; a rate per transfer should not be treated as proof that the test raises the overall chance of a live birth for every patient.
For context, ASRM’s 2024 opinion cites Society for Assisted Reproductive Technology (SART) figures showing that the proportion of US IVF cycles using PGT rose from 14% in 2014 to 44% in 2019. Those figures describe historical use, not current prevalence or evidence of clinical benefit.
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How embryo assessment fits into the transfer decision
Embryo ranking is one part of deciding what to transfer and when. The discussion may include whether to transfer at the cleavage stage or continue culture, how many embryos to transfer, and what to do with suitable embryos that are not transferred. A clinic may freeze suitable remaining embryos for possible future treatment, subject to their suitability and clinic policy.
HFEA describes elective single-embryo transfer as best practice for most women with more than one good-quality embryo, in part because transferring more than one can raise the risk of multiple birth. Individual recommendations differ with patient circumstances and local clinical practice. Ask your clinic how its embryo assessment, transfer recommendation, and plans for any remaining embryos fit together.
Quick Recap
Questions to take to your fertility clinic
- Which grading system does this clinic use, and what do my embryo’s number and letters mean in that system?
- What did the embryologist observe about development and morphology, and how did those observations affect the ranking?
- Why is the clinic recommending cleavage-stage transfer or continued culture to blastocyst in my circumstances?
- If PGT-A is being considered, what benefit does the clinic expect for me, what outcomes could be reported, and how would each affect transfer options?
- How does the clinic approach mosaic or no-result findings, and should I speak with a genetic counselor?
- How many embryos does the clinic recommend transferring, and what would happen to suitable embryos not transferred?
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