Poor fertilization in IVF is usually due to sperm, not the egg

A large 2025 study of donor egg cycles found that poor fertilization in IVF is uncommon, but when it does occur it’s usually caused by sperm rather than the egg. Most cases of low fertilization were sperm-related, while only a minority were linked to eggs or other factors.

ICSI usually achieves fertilization rates of 70–80%, but poor fertilization in IVF (≤30%) or even fertilization failure can still occur.

These cases are often assumed to be due to the egg, but studies make it hard to separate the effect of the egg and sperm to know for sure. Standard semen analysis is also limited in predicting outcomes like fertilization.

To clarify the role of sperm, a study by Torra-Massana et al. (2025) analyzed donor oocyte cycles, where eggs from the same young donor were split across different recipients.

The study looked at donor egg cycles where fertilization was low. They then checked what happened when eggs from the same donor were used with sperm from another man. If the same donor’s eggs fertilized well in another cycle, the problem was likely with the sperm, not the egg.

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Study details

• Study type: Retrospective cohort that took place between 2015–2022 in Spain
• Participants: 13,632 ICSI cycles (recipients) from 2,963 oocyte donors (ages 18–35).
• Study inclusions/exclusions: Recipients had 5 or more eggs from a donor to be included in the study; excluded testicular sperm, paternal karyotype abnormalities, assisted oocyte activation (AOA).
• Primary outcome: Incidence of sperm-related poor fertilization (≤30%)

Poor fertilization in IVF was uncommon, but linked mainly to sperm

In this study, eggs from the same donor were split between different patients and fertilized with sperm from different men. If one patient had poor fertilization (≤30%) and another had good fertilization (>65%), the difference was linked to the sperm and not the egg, since the eggs were from the same donor

They found poor fertilization (≤30%) in 3.7% of all donor ICSI cycles (510/13,632). Within this group:

• 429 cycles (84.1%) were clearly due to sperm, because the same donor’s eggs fertilized well with another man’s sperm.
• 81 cycles (15.9%) were not linked to sperm. These could be related to the eggs, lab procedures, or chance, but we can’t say for sure.

This means that 84.1% of poor fertilization cycles (≤30%) were linked to sperm, not the egg. When they looked at all the cycles, 3.1% (429/13,632) had poor fertilization related to the sperm.

Severely poor fertilization (≤10%) was very rare, seen in 0.6% of all cycles (78/13,632). Most of these were sperm-related (59/78, 75.6%) and 19 (24.4%) were not.

Of the 510 poor fertilization cases, only 23 (4.5%) showed major abnormalities on semen analysis, highlighting that routine semen testing is not reliable for predicting poor fertilization.

Other factors linked to low fertilization in IVF

The researchers also looked at other factors that might explain poor fertilization. They found that:

• Fertilization rates were lower when oocytes were frozen instead of fresh.
• Male age had a negative effect.
• Sperm concentration and motility didn’t predict poor fertilization.
• Use of partner or donor sperm didn’t predict outcomes.

Conclusions

This study found that poor fertilization in IVF was uncommon, happening in fewer than 4% of cycles. When low fertilization did occur, it was much more often linked to sperm (84%) than to eggs or other factors (16%).

Because young donor eggs were used, which can sometimes “fix” sperm problems due to DNA repair mechanisms, sperm-related fertilization issues may actually be more common in real-life IVF cycles with older patient eggs.

For patients, this means that while poor fertilization is rare, sperm problems are the main reason when it happens. About 3% of all cycles may show poor fertilization because of sperm, even if semen tests look normal. This highlights the need for better ways to detect hidden sperm issues, so that treatment plans don’t focus only on the egg when fertilization fails.

Addressing sperm issues may require tests beyond a standard semen analysis, such as assessing sperm DNA fragmentation or genetic variants like PLCZ1. In some cases, treatments such as assisted oocyte activation (AOA) or alternative sperm selection methods (eg. PICSI, Zymot) may be considered, though the evidence for their effectiveness is still limited.

Limitations of this study include that it was retrospective, used only donor eggs, and did not include detailed sperm data after preparation. Some of the non-sperm cases could also be due to technical issues in the lab.

Want to read more about poor fertilization in IVF and how it might be treated?

Related studies

These additional studies were referenced by the authors of the paper and haven’t been covered on Remembryo. They may be helpful if you’re exploring this topic further. This section is available for paid subscribers.

Reference

Torra-Massana M, Morse B, Miguel-Escalada I, Guillén JJ, Rodriquez-Aranda A, Popovic M, Sakkas D. When does sperm impact fertilization success? The incidence of sperm-related poor fertilization after ICSI in 13 632 matched oocyte donation cycles. Hum Reprod. 2025 Aug 5:deaf152. doi: 10.1093/humrep/deaf152. Epub ahead of print. PMID: 40794597.

 

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About Embryoman

Embryoman (Sean Lauber) is a former embryologist and the founder of Remembryo, an IVF research and fertility education website. After working in an IVF lab in the US, he returned to Canada and now focuses on making fertility research more accessible. He holds a Master’s in Immunology and launched Remembryo in 2018 to help patients and professionals make sense of IVF research. Sean shares weekly study updates on Facebook, Instagram, and Reddit regularly. He also answers questions on Reddit or in his private Facebook group.

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