Cleavage-Stage Embryos and Blastocysts: How the Two Stages Differ

Cleavage-stage embryo and blastocyst are not two kinds of embryo. They are two points on the same developmental path. An embryo that reaches the blastocyst stage has passed through the cleavage stage first. The practical differences are about timing, what the laboratory can see, and what can be done with the embryo at each point.

This guide explains the sequence, what is observable at each stage, and how those differences shape conversations about PGT and transfer. It does not tell you which stage is right for your treatment; that depends on your own cycle and your treating team’s assessment.

The short answer

A cleavage-stage embryo is typically observed around day 2 to day 3 after fertilisation. It has divided into a small number of cells, called blastomeres, and each of those cells can usually be seen individually. There is no fluid-filled cavity.

A blastocyst is typically observed around day 5 to day 6. The cells have compacted, a fluid-filled cavity has formed, and two distinct populations of cells have appeared: the inner cell mass, which is associated with the future fetus, and the trophectoderm, the outer layer associated with the placenta. These descriptions are general embryology; the exact day an individual embryo reaches a stage varies.

A day-by-day view of early development

The timeline below describes the usual sequence after fertilisation. It is a general pattern, not a schedule your embryos must follow.

• Day 0 to day 1: fertilisation occurs, and the resulting single-cell zygote begins dividing.
• Day 2 to day 3: the embryo is in the cleavage stage, with a small number of visible cells and no cavity.
• Day 3 to day 4: the cells begin to compact and adhere more tightly, a transition sometimes called morula formation.
• Day 5 to day 6: a cavity forms and the embryo becomes a blastocyst, with an inner cell mass and trophectoderm.

Some embryos develop more slowly or stop developing altogether. A laboratory report that says an embryo had not reached blastocyst by a given day is a description of that embryo at that moment, not a prediction about any future cycle.

Side-by-side comparison

Feature	Cleavage stage	Blastocyst stage
Typical timing	Around day 2 to day 3 after fertilisation	Around day 5 to day 6 after fertilisation
Visible structure	Individual cells (blastomeres), no cavity	Compacted cells, fluid-filled cavity, inner cell mass and trophectoderm
Typical reporting language	Cell number, symmetry, degree of fragmentation	Expansion or hatching description, inner cell mass and trophectoderm grading
What the laboratory can assess	Cell count and appearance of the visible cells	Overall structure and the appearance of the two cell populations
What the laboratory cannot assess	Chromosome number or genetic content from appearance alone; how the embryo would have developed later	Chromosome number or genetic content from appearance alone; whether an embryo would implant

Grading systems differ between laboratories, and the same embryo can be described in different words by different teams. If you are comparing reports, it helps to ask each laboratory to explain its own scale rather than assuming the numbers mean the same thing. A separate guide on comparing embryo grading explanations between IVF laboratories sets out what to check.

Why PGT biopsy is generally done at the blastocyst stage

PGT-A assesses chromosome number in cells sampled from an embryo. The sample is a biopsy, and the result describes the sampled cells, not the whole embryo with certainty. Testing has limitations and can produce incorrect findings, and a result does not guarantee a successful pregnancy or a healthy child.

At the blastocyst stage, the trophectoderm is a separate outer layer, so a biopsy can be taken from those cells while the inner cell mass remains. At the cleavage stage, every visible cell is still part of the same undifferentiated group, so removing a cell is a different proposition. This is why trophectoderm biopsy at blastocyst stage is the usual approach in PGT programmes.

Biopsy is not risk-free. Embryo biopsy can cause damage and prevent further development. How that risk applies in your situation is a question for your treating clinician and embryologist, not something a general article can quantify for you.

What happens if embryos do not reach blastocyst

Extended culture to blastocyst means waiting longer before transfer or freezing. Some embryos stop developing during that wait. If none of your embryos reach the blastocyst stage, there may be no trophectoderm sample to test and no blastocyst to transfer or freeze from that cycle. That outcome is a reason for a careful conversation with your clinician about what the laboratory observed and what options exist, not a verdict on future attempts.

Because extended culture can reduce the number of embryos available for transfer or freezing, clinics sometimes discuss transferring at cleavage stage instead. That trade-off is a clinical decision for your team, based on your history and the laboratory’s observations.

Vitrification and transfer timing

Embryos can be vitrified (fast-cooled) at either stage, and the stage at which an embryo is frozen is recorded in the laboratory report. Transfer timing follows from that: a cleavage-stage transfer happens earlier in the cycle than a blastocyst transfer, and a frozen transfer is scheduled separately from the retrieval cycle.

Whether a fresh or frozen transfer is planned after PGT is a separate decision with its own considerations. A comparison of fresh versus frozen embryo transfer after PGT covers the pathway differences without recommending one for you.

Questions worth putting to your own embryologist or clinician

These are clinical questions, and your treating team is the right place to ask them. A short written list helps you get specific answers rather than general ones.

• At which stage does this laboratory usually report embryos, and what does its grading scale mean?
• If extended culture is planned, on which day is blastocyst development assessed, and what happens to embryos that have not reached blastocyst by then?
• If PGT is planned, which cells are sampled, and what does the report describe?
• If few or no embryos reach blastocyst in a cycle, what will the team review with us afterwards?
• At which stage would any embryo be frozen, and how does that affect transfer scheduling?

You can also ask for the laboratory’s written embryo report and for an explanation of any term you do not recognise. Reports are clinical documents; interpretation belongs with your clinician.

Where coordination help fits

Understanding the stages is the clinical part. The practical part is making sure questions reach the right person and that records travel with you. A coordination service can help organise your existing records and a dated list of outstanding documents, request an appointment or remote consultation and confirm the format and any fee status, and assist with language and travel planning around confirmed appointments. After you return home, it can help request records and route follow-up questions to the hospital. It does not interpret results, diagnose, prescribe or promise an outcome, and appointment availability always depends on the hospital.

If you are at the stage of comparing what clinics include in their written quotations, the guide to what to confirm in writing about PGT-A testing cost in Thailand lists the items worth checking. For a broader look at how clinic success-rate claims are presented, see how to read clinic success-rate claims.

Sources: Embryo biopsy can cause damage and prevent further development. — HFEA PGT-A frequently asked questions (checked 2026-10-15).

Frequently asked questions

Does a blastocyst-stage embryo have a better chance than a cleavage-stage embryo?

That is a clinical question about your own cycle, and the answer depends on the embryos, your history and the laboratory. What can be said generally is that reaching blastocyst is a sign that an embryo has continued developing, but it does not guarantee implantation or a healthy pregnancy. Ask your treating clinician how the stage question applies to your situation.

Can PGT be done on a cleavage-stage embryo?

Trophectoderm biopsy at blastocyst stage is the usual approach because the outer cell layer can be sampled separately from the inner cell mass. Whether any alternative is appropriate in your case is a decision for your treating team and the laboratory, and it should be discussed with you directly rather than assumed.