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Before an IVF transfer, embryologists assess how embryos develop and what they look like under the microscope. They use those observations to rank embryos—not to predict with certainty which one will implant or lead to a baby. Some patients also consider PGT-A, a separate chromosome test that has limitations and is not recommended as routine screening for every IVF patient.
What embryologists assess
Embryo evaluation combines developmental stage and morphology: the embryo’s visible structure, cell organization and progress over time. The assessment helps a clinic compare embryos available in a particular cycle. It does not establish that an embryo has the right number of chromosomes or guarantee a pregnancy.
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Cleavage-stage embryos
At the cleavage stage, commonly assessed on day 2 or 3, embryologists may consider the number of cells, how quickly they have divided, whether the cells are evenly sized and whether cell fragments are present. These observations describe appearance and development; they cannot identify every chromosomal or developmental issue.
Blastocysts
Clinics may continue culturing embryos to the blastocyst stage, commonly reached on day 5 or 6. A blastocyst has developed a fluid-filled cavity and distinct cell groups. The inner cell mass (ICM) contributes to the fetus; the trophectoderm (TE) contributes to supporting tissues, including the placenta.
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How to read a blastocyst grade
A commonly used Gardner-style grade combines a number with two letter assessments. The number, from 1 to 6, describes expansion and hatching: an early blastocyst has a small cavity; a full blastocyst’s cavity fills the embryo; an expanded blastocyst has a larger cavity and a thinner outer shell; and later stages describe the embryo beginning to hatch from, or having hatched out of, that shell.
For stages 3 through 6, the first letter describes the ICM, including how many cells it has and how tightly they are grouped. The second describes the TE, including cell number and whether the cells form a cohesive layer. ASRM’s grading resource describes these features, but clinics may use different conventions. Ask your clinic to interpret the exact grade using its own laboratory’s system.
The updated ESHRE/ALPHA Istanbul Consensus, published in 2025, provides recommended static and dynamic morphology assessment criteria and guidance for ranking embryos. Morphology grading remains a subjective assessment. A grade can help prioritize embryos, but no grade or cutoff guarantees implantation, pregnancy or live birth.
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Day-3 transfer or continued culture to blastocyst?
Continuing culture gives the laboratory more time to observe which embryos keep developing, which can help rank them. It also creates a real trade-off: some embryos do not reach the blastocyst stage, so a patient with few embryos may have none available for transfer at that point. It is not possible to know whether a particular embryo that stopped developing in the laboratory would have continued to a successful pregnancy if transferred earlier, HFEA notes.
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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errors| Approach | What the clinic observes | Key consideration |
|---|---|---|
| Cleavage-stage transfer, commonly day 2 or 3 | Cell number, division timing and pattern, and fragmentation | Transfer can happen before the additional development observed during longer culture; the cited guidance does not establish that an embryo that does not reach blastocyst would have succeeded after an earlier transfer. |
| Continued culture to blastocyst, commonly day 5 or 6 | Which embryos continue developing and, at blastocyst, expansion, ICM and TE morphology | Provides more information for ranking, but some embryos may not reach transfer stage; this can matter especially when few embryos are available. |
The choice depends on the embryos available and the patient’s circumstances. Ask the fertility team what it expects to learn from continued culture in your specific cycle and what the plan would be if no embryo reaches blastocyst.
What PGT-A adds—and what it cannot tell you
Preimplantation genetic testing for aneuploidy (PGT-A) assesses chromosome number. 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. It is an additional selection tool, not the same as morphology grading and not a required step in every IVF cycle.
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Results may be reported as euploid, aneuploid, mosaic or no result. A mosaic result means the tested sample showed a mixture of cells with different chromosome findings. The proportion and interpretation matter, and clinics can differ in their reporting and transfer policies. Ask the clinic to explain what a result means for your options; a genetic counselor may also be helpful.
PGT-A does not promise that an embryo will implant or become a baby. Because the test uses a biopsy and a sample to represent the embryo, an inaccurate result or biopsy may mean a viable embryo is not available for transfer. Testing can also reduce the number of embryos available to transfer.
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Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteIn its 2024 committee opinion, the American Society for Reproductive Medicine (ASRM) states: “The value of PGT-A as a routine screening test for all patients undergoing in vitro fertilization has not been demonstrated.” ASRM says routine blastocyst biopsy with PGT-A in all infertile patients cannot currently be recommended. HFEA’s patient guidance says there is no randomized-trial evidence that blastocyst-stage PGT-A improves the chance of having a baby for most IVF patients. These positions do not rule out individual use; the possible benefit, limitations, alternatives and outcomes should be discussed in light of age, history, embryo number and priorities.
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Use of PGT in US IVF cycles rose from 14% in 2014 to 44% in 2019, according to historical Society for Assisted Reproductive Technology data cited in ASRM’s 2024 opinion. Those figures describe use in those years, not current prevalence or evidence that testing improves outcomes.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How embryo evaluation informs transfer decisions
Embryo ranking is one part of planning a transfer. The team and patient also consider the transfer stage, how many embryos to transfer and whether suitable embryos not transferred should be frozen for future treatment.
How many embryos to transfer
HFEA describes elective single-embryo transfer as best practice for most women with more than one good-quality embryo, partly to reduce the risk of multiple birth. The appropriate plan depends on individual circumstances and local clinical practice.
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What happens to embryos not transferred
Suitable embryos may be cryopreserved for a later attempt, subject to their suitability and the clinic’s policy. Ask which embryos the clinic considers suitable for freezing and what options it offers for their future use.
Questions to ask your fertility team
- What does this grade mean in your laboratory’s grading system, and how did you assess the embryo?
- What are the advantages and risks of transferring at the cleavage stage versus continuing culture in my situation?
- If PGT-A is being considered, what result categories might I receive, how would each affect transfer options, and what are the alternatives?
- How many embryos do you recommend transferring, and what could happen to suitable embryos not transferred?
HFEA’s embryo-decision guidance was last reviewed on June 2, 2026. Recommendations, laboratory protocols and regulation can vary by country and clinic, so interpret general guidance with your fertility team.
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