By Dr. Ram Prakash, Maaeri Fertility & IVF Centre
Do you know that embryo freezing was once treated as nothing more than a backup option? It is nothing of the sort today. It has worked its way right to the centre of how IVF is practised, and it shifts three things at the same time, which are your success rates, your safety outcomes and the flexibility which you have in building a family. So what does embryo freezing actually involve, when do we reach for it, and what does the evidence really say about frozen transfers when they are set against fresh ones? Let us go through all three of these, because taken together they explain the whole shift.
What Is Embryo Freezing and How Has the Technique Evolved?
Embryo freezing, which is also called embryo cryopreservation, means holding fertilised embryos at ultra-low temperatures so that they can be used in a later treatment cycle. Vitrification is the method which is used today.
It is a flash-freeze which cools the embryo so fast that ice crystals never get the chance to form at all, and it keeps the cell integrity intact in a way which the older slow-freeze methods never managed to. Survival rates of 90 to 95% per embryo are standard now in an experienced laboratory, whereas slow-freeze only ever reached 70 to 80%, and that gap is the whole reason the technique changed.
Moreover, embryos can go into the freeze at the cleavage stage, which is day 2 to 3, or at the blastocyst stage, which is day 5 to 6. Wherever it is possible we prefer blastocyst vitrification, and the reason behind this is simple enough. Only the embryos which have the developmental potential to reach day 5 actually get that far in the first place, so each one which is transferred afterwards is a considerably better bet for you.
How Does Frozen Embryo Transfer Compare to Fresh Transfer?
A meta-analysis of nine randomised controlled trials00111-9/fulltext) found a significantly higher live birth rate after elective FET than after a fresh transfer, with a relative risk of 1.12. The neonatal outcomes point in the same direction as well. A 2024 meta-analysis of 171,481 participants found that fresh transfers carry higher rates of preterm birth (OR 1.26), low birth weight (OR 1.37), and small-for-gestational-age infants (OR 1.81).
| Factor | Fresh Embryo Transfer | Frozen Embryo Transfer |
| Timing | Same stimulation cycle | Separate cycle after recovery |
| Endometrium | Affected by stimulation hormones | Natural or medicated, with no stimulation effect |
| OHSS risk | Present, especially in high responders | Virtually eliminated |
| Preterm birth risk | Higher | Lower |
| Cumulative live birth | Comparable or slightly lower | Comparable or slightly higher |
| Suitable when | Mild stimulation, normal response | High response, PCOS, PGT needed, thin lining |
So why should this be the case? Ovarian stimulation works on the endometrial environment, and the high oestrogen levels change the uterine receptivity at precisely the moment when the transfer is happening. However, if you run a frozen cycle instead, with the embryo transfer taking place later on in a calmer cycle, then that whole variable simply drops out of the equation.
Who Benefits Most from a Freeze-All Approach?
Freeze-all means that every embryo is vitrified and every transfer is deferred. Is it meant for everybody? It is not. However, for certain groups of patients it is the approach which we do recommend.
PCOS and High Responders
Women who have PCOS or a high ovarian reserve produce follicles in very large numbers, and this is what puts them at a meaningful risk of OHSS. Going straight on to a fresh transfer after a high-response cycle only piles onto that risk instead of easing it, and freeze-all is what takes it off the table altogether.
The egg retrieval proceeds exactly the way it normally would, and the body then gets the time it needs to recover before any transfer happens. Patients who are on minimal stimulation protocols need this less often, as their response is more controlled right from the outset.
PGT Testing Candidates
Embryo freezing is what preimplantation genetic testing actually runs on. The embryos are biopsied and then frozen, and they are held while the results come back, which usually takes anywhere from one to three weeks. The thing to note here is that freeze-all is what allows the right embryo to be chosen before a transfer rather than after a disappointing one.
Thin Endometrium or Poor Lining Response
Wherever the lining has not built up adequately, and below 7 mm is the usual threshold for concern, pushing on with a fresh transfer risks a failure which has nothing whatever to do with the embryo itself. It is recommended that you freeze instead, and then come back in a dedicated cycle with focused endometrial preparation, so that both the embryo and the lining get the best conditions which we are able to give them.
How Long Can Frozen Embryos Be Stored?
Vitrified embryos hold up over very long periods, and pregnancies have been reported from embryos which were stored for more than a decade. Under India’s ART (Regulation) Act 2021, the storage runs to five years, with an extension which is available on application. For the great majority of IVF patients that is more than ample, and it leaves you genuine room to plan a family at whatever pace suits you.
Frequently Asked Questions
Does freezing damage the embryo?
With modern vitrification the risk is very low indeed. Survival rates of 90 to 95% per embryo are standard in experienced laboratories, and moreover every single embryo is assessed after warming, before any transfer goes ahead.
Is a frozen embryo transfer less natural than a fresh transfer?
Not inherently, no. The endometrial environment in a frozen cycle is often the more natural one of the two, because it is not sitting under the influence of the hormones which stimulation generates.
Can all embryos be frozen?
Not all of them reach a stage which suits vitrification. Only the embryos which develop to the blastocyst, or to a good-quality cleavage stage, go into the freeze. At Maaeri the development gets monitored very closely, so that only those embryos which are most likely to survive the thaw and implant successfully are the ones which are kept.
Embryo freezing is not a contingency plan at all. It is a clinically meaningful strategy which improves your safety, widens your options, and in plenty of patient groups improves the outcome as well. At Maaeri the call on whether to freeze all of them, freeze some of them or go ahead fresh is made individually, and it is made off each patient’s own response, diagnosis and the specific circumstances of that particular cycle.
This article is for general educational purposes and is not a substitute for personalised medical advice from your fertility specialist or embryologist.