PGT on Frozen Embryos: How It Works and What to Ask

Yes, embryos that were already frozen can be thawed, biopsied for preimplantation genetic testing and refrozen for later transfer. It is done, and it means patients who stored embryos before deciding about testing are not shut out of it. It does add a thaw and a refreeze, so it is a decision with a cost attached rather than a free extra. Before agreeing to it, the more important question is what the testing itself is expected to achieve, because for PGT-A the evidence on that is not what most patients assume.

What is PGT?

PGT examines embryos before transfer. There are three types, answering different questions:

• PGT-A counts chromosomes, looking for missing or extra ones.
• PGT-M looks for one specific single-gene condition the parents are known to carry.
• PGT-SR looks for structural rearrangements such as translocations.

Our article on the difference between PGT-A, PGT-M and PGT-SR covers each in detail. The distinction matters here because PGT-M and PGT-SR test for something already identified, whereas PGT-A is a screen, and the evidence for the two is not the same.

How does testing frozen embryos work?

1. The embryos are thawed under controlled conditions.
2. A small sample of cells is taken from each, at the blastocyst stage, from the cells that would form the placenta rather than the baby.
3. The samples go to a genetics laboratory.
4. The embryos are refrozen while the results are awaited.
5. Once results are back, transfer is planned accordingly.

The additional handling is the trade-off compared with testing before the first freeze. Vitrification has made embryos more robust through freeze and thaw cycles than older slow-freezing methods allowed, but not every embryo survives every thaw, and that is a question to put to your own clinic rather than to take from a general figure.

What does the evidence say about PGT-A?

This is the part worth reading before deciding. The HFEA rates PGT-A red for improving the chance of having a baby for most patients, meaning there is no evidence from randomized controlled trials that it does so, and it notes that using PGT-A "may decrease the chance of having a baby" because it is a selection tool that reduces the number of embryos available for transfer. It rates PGT-A green for reducing the chance of miscarriage, and grey in older women, meaning there is not enough good-quality evidence either way in that group.

Both of those can be true at once. Fewer miscarriages does not automatically mean more babies. If you have a limited number of frozen embryos, the point about reducing the number available for transfer applies with particular force.

For PGT-M and PGT-SR, where a specific genetic or chromosomal finding is already known in the family, the case is different and does not rest on this evidence.

Testing before or after freezing?

	Before freezing	After freezing
Handling steps	Fewer	Additional thaw and refreeze
Timing	Requires coordination within the cycle	Flexible, can be done years later
Applies to	Embryos not yet frozen	Embryos already in storage

In practice the choice is usually made for you by where you already are. If the embryos are already frozen, testing after thawing is the only option.

What to ask us

• What is our laboratory's embryo survival rate through thaw, biopsy and refreeze?
• How many embryos do I have, and what happens if testing leaves me with none suitable for transfer?
• What specifically is the testing expected to change about my treatment?
• How long will results take, and what does it add to the cost?
• What happens if a result is inconclusive?

The second question is the one most often left unasked, and it is the one the HFEA's red rating turns on.

We at Millennium IVF Clinic will go through whether testing applies to your embryos and what it would mean. You can arrange a consultation with our team.

Frequently asked questions

Does thawing and refreezing damage embryos?

Vitrification is designed to minimize the damage older methods caused, but not every embryo survives every thaw. Ask us for our own laboratory’s survival figures rather than relying on a general claim.

Will PGT-A improve my chance of a baby?

The HFEA rates it red on that question for most patients and notes it may reduce the chance. It rates it green for reducing miscarriage. Discuss what that means for your specific situation.

Can old embryos still be tested?

Embryos frozen years ago can generally be thawed and tested. Whether it is advisable depends on how many you have and what you are hoping the result will change.

What if testing leaves no transferable embryos?

This is a real possibility and the reason to ask about it in advance. Establish with your specialist what your options would be before testing begins, not after.