IVF Attrition Explained: Where Eggs and Embryos Are Lost During Treatment
Twelve eggs are retrieved during IVF. Nine are mature. Six fertilise. Three reach the blastocyst stage.
Watching those numbers fall can be one of the most confronting parts of fertility treatment, particularly when the initial egg retrieval created expectations of having many embryos available.
But this progressive reduction—often called IVF attrition—does not automatically mean something has gone wrong. It reflects the multiple biological stages an egg must successfully pass through before becoming an embryo potentially suitable for transfer.
IVF attrition is the reduction in numbers between egg retrieval, egg maturity, fertilisation and embryo development. Not every follicle produces an egg, every retrieved egg is not necessarily mature, every mature egg will not fertilise, and every fertilised egg will not reach the blastocyst stage. The amount of attrition varies considerably between patients and cycles.
Why Do IVF Numbers Drop So Quickly?
IVF makes reproductive biology visible.
During unassisted conception, eggs may not fertilise and fertilised eggs may stop developing without anyone knowing it happened. IVF moves several of these stages into the laboratory, where each can be observed and counted.
The American Society for Reproductive Medicine (ASRM) notes that approximately 70% of mature eggs typically fertilise during IVF and around half of fertilised eggs subsequently develop to the blastocyst stage. These are broad population estimates, not predictions for an individual treatment cycle.
This means the number of eggs retrieved should never be interpreted as the number of embryos a patient will ultimately have.
Each stage has a different biological requirement.
Stage 1: Follicles Do Not Always Equal Eggs
During ovarian stimulation, ultrasound is used to monitor follicles developing within the ovaries.
If 12 follicles are visible before retrieval, it is easy to mentally convert that into 12 eggs.
The numbers are not necessarily identical.
A follicle is the fluid-filled ovarian structure in which an oocyte—an egg cell—develops. Follicles can differ in size and developmental stage, and not every follicle necessarily yields an egg during retrieval.
The number of eggs collected can therefore be lower than the follicle count seen on ultrasound.
This is the first potential point of attrition.
Stage 2: Not Every Retrieved Egg Is Mature
An egg also needs to reach the appropriate developmental stage before it can normally fertilise.
For IVF and intracytoplasmic sperm injection (ICSI), the laboratory assesses egg maturity. A mature egg has generally reached metaphase II (MII).
Some retrieved eggs may still be immature.
This creates an important distinction between eggs retrieved and mature eggs available for fertilisation.
For example, retrieving ten eggs does not necessarily mean ten eggs can undergo ICSI.
Egg maturity can be influenced by follicular development, ovarian stimulation, timing of the trigger medication and individual biological variation.
Stage 3: Not Every Mature Egg Fertilises
Fertilisation creates another major reduction.
In conventional IVF, sperm and eggs are placed together in the laboratory. With ICSI, an embryologist injects a single sperm into a mature egg.
Neither technique guarantees fertilisation.
Normal fertilisation is generally identified by the appearance of two pronuclei containing maternal and paternal genetic material.
ASRM estimates that approximately 70% of mature eggs fertilise during IVF.
Individual results can be considerably higher or lower.
Egg factors, sperm factors and other biological variables can influence this stage.
This is also why unexpectedly low fertilisation should be considered separately from overall IVF attrition. If very few—or none—of the mature eggs fertilise, the fertility team may investigate that particular stage rather than describing the entire cycle simply as unsuccessful.
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Stage 4: Fertilised Eggs Can Stop Developing
Successful fertilisation does not guarantee continued embryo development.
After fertilisation, an embryo begins dividing. By approximately day three it is generally described as a cleavage-stage embryo.
Some embryos continue developing normally.
Others stop.
This is known as embryonic arrest.
Embryo development requires a highly coordinated sequence of cellular and genetic processes. An embryo can therefore fertilise normally but fail to continue developing.
This explains why a promising fertilisation report can be followed several days later by a much smaller number of developing embryos.
Stage 5: Not Every Embryo Reaches Blastocyst
A blastocyst is an embryo that has usually developed for approximately five or six days after fertilisation and reached a more advanced cellular structure.
ASRM estimates that approximately half of fertilised eggs typically continue to blastocyst.
Again, this is an average rather than an individual target.
Some patients may have a high blastocyst conversion rate. Others may have substantially fewer embryos reach this stage.
Clinicians also cannot predict with certainty which early embryos will ultimately develop into viable blastocysts.
That uncertainty is part of IVF biology.
Why Does Age Matter?
Age can affect the IVF funnel because egg number and chromosomal competence are different issues.
As maternal age increases, the probability of chromosomal abnormalities in embryos generally increases.
An embryo with an abnormal chromosome number is described as aneuploid. An embryo with the expected chromosome complement is described as euploid.
Age-related aneuploidy is an important contributor to implantation failure and miscarriage.
This means two women who retrieve the same number of eggs can ultimately have very different embryo outcomes.
Egg quantity is therefore only one part of IVF prognosis.
Does PGT-A Cause Another Drop in Embryo Numbers?
For patients using preimplantation genetic testing for aneuploidy (PGT-A), another selection stage may occur after blastocyst development.
Appropriate blastocysts can be biopsied and assessed for chromosome copy-number abnormalities. Results may include euploid, aneuploid or more complex findings such as mosaic results.
However, PGT-A is not a universal requirement in IVF.
ASRM states that the value of PGT-A as routine screening for every IVF patient has not been established. Whether it is appropriate depends on individual circumstances and should be discussed separately with the fertility team.
Does High IVF Attrition Mean Something Went Wrong?
Not necessarily.
The most important question is where the attrition occurred.
Few mature eggs raise different questions from poor fertilisation.
Poor fertilisation is different from repeated embryo arrest.
And producing blastocysts that subsequently fail to implant represents another clinical issue again.
Rather than focusing only on the first and final numbers, ask for the results at each transition:
retrieved eggs → mature eggs → normally fertilised eggs → developing embryos → blastocysts
This makes it easier to identify whether the cycle followed a broadly expected pattern or whether one particular stage produced an unexpected result.
Can the Next IVF Cycle Be Different?
Yes.
A previous cycle provides useful information about ovarian response, egg maturity, fertilisation and embryo development, but it does not guarantee identical results in another cycle.
Egg yield can change. Maturity can change. Fertilisation can change. Embryo development can change.
Clinicians may also adjust treatment based on information learned during an earlier cycle.
This variability is one reason comparing your IVF numbers with another patient’s results can be misleading.
Age, diagnosis, ovarian reserve, sperm factors, treatment protocols and other circumstances may differ considerably.
When to Discuss IVF Attrition With Your Fertility Specialist
Some attrition is expected.
A more detailed review may be particularly useful when no mature eggs are obtained, fertilisation is substantially lower than expected, fertilisation fails completely, no embryos reach blastocyst or a similar pattern occurs repeatedly across treatment cycles.
Ask your fertility specialist which stage produced the unexpected result and whether it represents normal biological variation or something requiring further investigation.
A more useful question than “Why did we lose so many embryos?” is:
“At which stage did our results differ from what you expected for my circumstances?”
That focuses the discussion on information that may actually influence future treatment.
IVF Attrition Is a Funnel, Not a Straight Line
The emotional difficulty of attrition often comes from mentally carrying the first number through to the end.
Twelve eggs can feel like 12 potential embryos.
Biologically, that is not what the number represents.
Retrieval creates a starting population. The eggs must then pass through maturity, fertilisation and continued embryo development before reaching blastocyst.
The numbers narrow because human reproduction itself involves substantial biological selection.
Understanding this does not make seeing embryo numbers fall easy.
But it does change what those numbers mean.
IVF attrition is not automatically evidence that your body failed or that treatment failed. What matters clinically is where attrition occurred, whether it was unexpected for your circumstances and what that information may reveal about the treatment cycle.
FAQs
How many retrieved eggs usually fertilise during IVF?
ASRM estimates that approximately 70% of mature eggs typically fertilise. This is a broad population estimate and individual results vary.
How many fertilised eggs usually reach blastocyst?
Approximately half of fertilised eggs may reach the blastocyst stage according to ASRM estimates, but outcomes vary considerably between patients and cycles.
Is it normal for embryos to stop developing before day 5?
Yes. Some embryos stop developing before reaching blastocyst. The clinical significance depends on the individual cycle and whether an unexpected pattern occurs repeatedly.
Does getting lots of eggs mean I will get lots of embryos?
No. Retrieved eggs must also be mature, fertilise normally and continue developing. Egg retrieval numbers therefore cannot predict the final number of blastocysts on their own.
Does IVF attrition mean poor egg quality?
Not automatically. Attrition can occur at several stages and may involve multiple biological factors. Identifying the specific stage where unexpected attrition occurred is more informative than the overall reduction.
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Sources
- American Society for Reproductive Medicine — It Takes More Than One: Why IVF Patients Often Need Multiple Embryos to Have a Baby.
- American Society for Reproductive Medicine — Blastocyst Culture and Transfer in Clinically Assisted Reproduction.
- American Society for Reproductive Medicine — guidance on preimplantation genetic testing for aneuploidy.
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