Low or Failed Fertilization in IVF: Causes and Next Steps
The day-one fertilization call can feel like the cycle has ended before it really started. Here's what the numbers mean, what typically causes low or zero fertilization, and what tends to change next time.
Fertilization is judged about 17 hours after insemination or ICSI by looking for two pronuclei (2PN), one from the egg and one from the sperm. The ESHRE/Alpha "Vienna consensus" lab benchmarks are:
- ICSI: at least 65% of injected mature eggs normally fertilized is the minimum competency; 80% or more is the benchmark.
- Conventional IVF: at least 60% of inseminated eggs is the minimum competency; 75% or more is the benchmark. Fewer than 5% of conventional IVF cycles should have no fertilization at all.
Total fertilization failure after ICSI affects about 1–3% of ICSI cycles. When it happens, the leading suspected cause is a failure of egg activation, often linked to the sperm protein PLC-zeta, though egg-side genetic causes also exist. ESHRE recommends artificial oocyte activation for complete or very low (<30%) fertilization after ICSI and for globozoospermia, but not as routine care.
In this article
How to read your fertilization report
Most embryology reports give a chain of numbers. Each one uses a different denominator, which is why people often think their results are worse than they are.
| Term on the report | What it means |
|---|---|
| Eggs or COCs retrieved | Cumulus-oocyte complexes collected at retrieval. Not every one contains a mature egg. |
| MII (mature) | Eggs that have reached metaphase II and can be fertilized. Only these are injected in ICSI. |
| MI / GV (immature) | Eggs not yet mature. They usually can't be fertilized normally. |
| 2PN | Normal fertilization: two pronuclei visible. |
| 1PN / 3PN | Abnormal fertilization patterns. These embryos are usually not used. |
| Degenerated | Eggs that didn't survive handling or injection. |
The Vienna consensus defines the ICSI fertilization rate as 2PN eggs divided by mature eggs injected, and the conventional IVF rate as 2PN eggs divided by all eggs inseminated. It also treats a maturity rate of 75% to 90% of retrieved eggs as the expected range. So "10 eggs, 5 fertilized" might mean a 50% rate. Or it might mean 5 of 6 mature eggs fertilized, which is excellent.
Small numbers swing hard
With three mature eggs, one that doesn't fertilize drops your rate to 67%. Two drop it to 33%. A single low-rate cycle with few eggs is often more about chance than a problem. A pattern across cycles, or zero fertilization with plenty of mature eggs, deserves a closer look.
What counts as "low"
Labs use the 2017 Vienna consensus from ESHRE and Alpha Scientists in Reproductive Medicine as a quality yardstick. It sets two levels for each key indicator: competency, the minimum a lab should reach, and benchmark, best practice.
| Indicator | Competency | Benchmark |
|---|---|---|
| ICSI normal fertilization (2PN ÷ mature eggs injected) | ≥65% | ≥80% |
| Conventional IVF normal fertilization (2PN ÷ eggs inseminated) | ≥60% | ≥75% |
| Cycles with failed fertilization (conventional IVF) | <5% | — |
| Eggs damaged during ICSI | ≤10% | ≤5% |
Source: ESHRE SIG Embryology and Alpha Scientists in Reproductive Medicine, The Vienna consensus, Reprod Biomed Online 2017.
These benchmarks describe a lab's performance across many patients. They aren't a pass mark for your cycle. Still, they give you a sense of scale. If your rate sits well below these figures across more than one cycle, or if it's zero, there is usually a reason worth looking for.
The main causes
Egg maturity
Immature eggs (MI or GV) generally can't be fertilized normally. A cycle with many retrieved eggs but few mature ones often looks like a fertilization problem when it's really a maturation or trigger-timing problem. That puts it in the same family as empty follicle syndrome, which is the far end of the same spectrum.
Sperm getting into the egg
In conventional IVF, sperm have to bind to the egg's outer shell (the zona pellucida) and penetrate it on their own. Low counts, poor motility, or binding problems can mean little or no fertilization, even when a semen analysis didn't look alarming. ICSI bypasses this step by injecting one sperm directly into each mature egg.
Egg activation after sperm entry
Getting the sperm inside the egg isn't the end of the story. The sperm has to trigger a series of calcium waves that "activate" the egg so it completes division and forms pronuclei. A 2023 review in Human Reproduction Update (Campos, Sciorio, and Esteves) identifies oocyte activation deficiency as the leading cause of total fertilization failure after ICSI, mainly linked to a deficiency of the sperm protein phospholipase C zeta (PLCζ). Men with globozoospermia, a rare condition with round-headed sperm, often lack PLCζ and frequently fail to fertilize even with ICSI.
The same review notes that some cases are on the egg side. Variants in genes including WEE2, PATL2, TUBB8, and TLE6 can disrupt activation even when the calcium signal is normal.
Lab and technical factors
Culture conditions, injection technique, and handling all affect results. That's why lab-wide statistics exist. It's fair to ask your clinic how your result compares with its own typical rates.
If conventional IVF failed
If you had low or zero fertilization with conventional IVF, the usual next step is ICSI for some or all eggs in the next cycle, since it removes the sperm-entry barrier. Some clinics split eggs between the two methods to learn more.
ESHRE's 2023 add-ons recommendations make a useful distinction. ICSI used because of a specific indication, such as previous fertilization failure or male factor, is standard care. ICSI used routinely for everyone should be thought of as an add-on, especially when it costs extra. After a fertilization failure, ICSI is clearly indicated.
Some labs also offer "rescue ICSI," injecting eggs that haven't fertilized by the next morning. If yours does, ask about its own results with it, because eggs age quickly after retrieval.
If ICSI failed: activation problems
Total fertilization failure after ICSI is uncommon, about 1–3% of ICSI cycles according to Campos and colleagues. If it happened with a reasonable number of mature eggs, the question shifts from "did sperm get in?" to "did the egg activate, and if not, is the problem on the sperm side or the egg side?"
Specialized centers can use tests that help locate the problem. The Campos review describes:
- Mouse oocyte activation test (MOAT): sperm are injected into mouse eggs to see whether they can trigger activation. Results are grouped as low, diminished, or normal activation capacity. A normal result points more toward an egg-side cause.
- Calcium analysis in mouse or, more rarely, human eggs.
- PLCζ testing and genetic screening for known sperm- or egg-side variants.
Not every clinic offers these, and you may not need them all. They're most useful when failure repeats or when the choice of treatment depends on the answer.
Artificial oocyte activation: evidence and caveats
Artificial oocyte activation (AOA) usually uses a calcium ionophore to mimic the calcium signal a normal sperm would provide. ESHRE's 2023 good practice recommendations say:
- AOA is not recommended for routine use.
- It is recommended for complete activation failure (0% 2PN), very low fertilization (below 30%), and globozoospermia.
ESHRE's review of the evidence found a meta-analysis of 14 studies in which ionophore AOA was associated with higher live birth rates overall. The effect was larger in people with previous fertilization failure or low fertilization. The analysis also showed substantial differences between studies and signs of publication bias. ESHRE found no increase in birth defects compared with ICSI alone, but stressed the need for long-term follow-up of children.
The Campos review adds that conventional calcium-based AOA works best when the problem is PLCζ-related on the sperm side. Egg-side problems may not respond, and the alternatives for those are experimental. It also notes mainly animal research suggesting AOA could cause epigenetic changes, and concludes AOA should be used judiciously, with counseling, as an innovative rather than established treatment.
In practice
If you've had complete or very low fertilization after ICSI with enough mature eggs, AOA in the next cycle is a standard, guideline-supported conversation. If your fertilization was "a bit lower than hoped" with few eggs, AOA is an add-on you probably don't need.
Building the next-cycle plan
A good next-cycle plan after low fertilization usually covers:
- Maturity: was the problem immature eggs? If so, review trigger type and timing first.
- Method: ICSI instead of or alongside conventional IVF, and AOA if indicated by the guideline criteria above.
- Sperm: a repeat semen analysis and, where relevant, a urology evaluation. Our guide to male fertility supplements covers what the evidence does and doesn't support. Testing like sperm DNA fragmentation sometimes enters the conversation, but it's a separate question from fertilization failure.
- Diagnostics: MOAT, PLCζ, or genetic testing if failure is complete or repeated.
- Fallbacks: if a severe egg-side or sperm-side activation defect is confirmed and doesn't respond, donor gametes become part of the discussion.
If you're considering moving your next cycle to a different clinic, at home or abroad, bring your complete embryology report, not just a summary. If Colombia is on your list, colombianivf.com is a resource for researching IVF there. Ask any lab you're considering about its ICSI fertilization rate and whether it offers AOA when indicated.
Questions for your embryologist
- How many eggs were mature, and what was my fertilization rate per mature egg?
- How does that compare with your lab's typical rate for ICSI or conventional IVF?
- Did you see any 1PN or 3PN eggs, or any degenerated after injection?
- Does this look more like a maturity, sperm-entry, or activation problem?
- Would you recommend ICSI, AOA, or any diagnostic testing next time, and why?
- Is a different trigger or stimulation approach worth considering?
Frequently asked questions
What is a normal fertilization rate for ICSI?
The ESHRE and Alpha Vienna consensus sets a minimum competency of 65 percent of injected mature eggs showing normal fertilization, with 80 percent or more as the benchmark. These are lab-wide quality targets, so a single cycle with few eggs can fall below them by chance.
How common is total fertilization failure?
After ICSI, total fertilization failure affects about 1 to 3 percent of cycles according to a 2023 review in Human Reproduction Update. For conventional IVF, the Vienna consensus expects fewer than 5 percent of cycles to have no fertilization.
What causes zero fertilization after ICSI?
The leading cause is failure of the egg to activate after sperm injection, most often linked to a deficiency of the sperm protein PLC-zeta. Egg-side genetic variants, immature eggs, and technical factors can also play a role.
What is artificial oocyte activation and do I need it?
Artificial oocyte activation uses agents such as a calcium ionophore to mimic the signal sperm normally provide. ESHRE does not recommend it routinely but does recommend it after complete fertilization failure, very low fertilization below 30 percent, or globozoospermia.
Should we switch from IVF to ICSI after poor fertilization?
After low or failed fertilization with conventional IVF, ICSI is the usual next step because it bypasses the need for sperm to penetrate the egg on their own. Your clinic may use ICSI for all eggs or split them between methods.
Sources
- ESHRE Special Interest Group of Embryology and Alpha Scientists in Reproductive Medicine. The Vienna consensus: report of an expert meeting on the development of ART laboratory performance indicators. Reprod Biomed Online. 2017. https://www.eshre.eu/-/media/sitecore-files/SIGs/Safety-and-quality/The-Vienna-consenus_Lab-KPI.pdf
- Campos G, Sciorio R, Esteves SC. Total fertilization failure after ICSI: insights into pathophysiology, diagnosis, and management through artificial oocyte activation. Hum Reprod Update. 2023;29(4):369-394. https://academic.oup.com/humupd/article/29/4/369/7087123
- ESHRE Add-ons working group (Lundin K, et al.). Good practice recommendations on add-ons in reproductive medicine. Hum Reprod. 2023. https://academic.oup.com/humrep/article/38/11/2062/7255766
Medical disclaimer: This article is general education, not medical advice, and it cannot account for your history, test results, or treatment plan. Please make medication and treatment decisions with your own fertility team, and never start, stop, or change a prescribed medication without talking to the prescriber.
If you have severe abdominal pain, heavy bleeding, fainting, or shoulder-tip pain during or after fertility treatment, seek urgent care.