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Fertility · Clinical Education · v2026.3
The IVF Attrition Funnel
Definition
IVF Attrition Funnel is defined as: the progressive biological reduction that occurs at each stage of IVF: from retrieved eggs to fertilised embryos to blastocysts to genetically tested embryos to transferred embryos and, ultimately, to live birth. Attrition is not a complication. It is an intrinsic property of human reproduction made visible by IVF.
IVF is often misunderstood as a linear process where retrieved eggs become embryos and embryos become pregnancies. In reality, human reproduction is biologically inefficient. At every stage of IVF development some eggs and embryos stop progressing. This progressive reduction is known as the IVF attrition funnel.
Understanding the attrition funnel helps intended parents interpret IVF outcomes more realistically and understand why clinics often aim to retrieve multiple eggs in a single cycle.
Author: Julio Gaggia
Stage 1. Eggs Retrieved
During an IVF cycle the ovaries are stimulated to produce multiple eggs. The number of eggs retrieved varies widely depending on age, ovarian reserve, and stimulation response.
Not all eggs retrieved are mature or capable of fertilisation. The first reduction in the IVF funnel occurs at this stage.
Stage 2. Fertilisation
After retrieval, eggs are fertilised with sperm using conventional insemination or intracytoplasmic sperm injection (ICSI).
Not every egg fertilises successfully. Some eggs fail to fertilise, while others fertilise abnormally and cannot continue development. The fertilisation rate is typically 60–80%.
Stage 3. Embryo Development
Fertilised eggs begin dividing and developing into embryos. During the first several days of development some embryos stop progressing due to biological errors in cell division.
Only a portion of fertilised eggs will continue developing toward the blastocyst stage.
Stage 4. Blastocyst Formation
The blastocyst stage typically occurs around day five or six of embryo development. At this stage embryos have formed the structures required for potential implantation.
Many embryos do not reach this stage. The blastocyst stage represents another narrowing point in the IVF funnel.
Stage 5. Genetic Viability
Even embryos that reach the blastocyst stage may contain chromosomal abnormalities. Some clinics perform preimplantation genetic testing (PGT-A) to identify embryos with the correct number of chromosomes.
Genetically normal (euploid) embryos have a higher probability of implantation, but they still do not guarantee pregnancy.
Stage 6. Embryo Transfer and Implantation
A selected embryo is transferred into the uterus. Successful implantation requires biological coordination between embryo quality and uterine receptivity.
Even strong embryos may not implant, which is why IVF success is often measured cumulatively across multiple cycles.
Example of an IVF Attrition Funnel
Example numbers from a typical IVF cycle might look like this. Actual outcomes vary widely depending on age, biology, and clinic protocols.
Visual Reference
IVF Attrition Funnel Diagram
Eggs retrieved → Fertilisation → Embryo development → Blastocyst stage → Genetically normal embryos → Embryo transfer
Why Clinics Aim for Multiple Eggs
Because attrition occurs at each stage, clinics typically aim to retrieve multiple eggs in a cycle. The goal is to increase the probability of producing at least one viable embryo for transfer.
Understanding this biological funnel can help intended parents interpret IVF statistics and treatment expectations more clearly.
Conclusion
The IVF attrition funnel reflects the biological reality of human reproduction. Not every egg becomes an embryo and not every embryo becomes a pregnancy. Recognising this progression helps explain why IVF outcomes vary and why fertility treatment is often measured across multiple attempts rather than a single cycle.
Knowledge Graph
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Sources
- SART National Summary Report (2023), https://www.sart.org
- ASRM: Optimising natural fertility, https://www.asrm.org