Introduction
For decades, Intracytoplasmic Sperm Injection (ICSI) has been the gold standard for treating severe male factor infertility. Since its clinical introduction in the early 1990s, ICSI has revolutionized the management of oligospermia and azoospermia. However, recent years have seen a significant trend toward the routine use of ICSI for all patients, regardless of sperm parameters—a practice often driven by the desire to minimize the risk of total fertilization failure. Despite its widespread adoption, evidence supporting the superiority of ICSI over conventional IVF (c-IVF) in cases without severe male factor infertility remains scarce. A secondary analysis of the INVICSI trial now provides critical insights into how these two fertilization methods affect early embryo development, morphokinetics, and overall embryo quality.
Highlights
Key clinical takeaways from the INVICSI secondary analysis include:
- ICSI resulted in a significantly lower number of high-quality blastocysts on Day 5 compared to conventional IVF in patients without severe male factor infertility.
- There were no significant differences in the timing of embryo development (morphokinetics) or cleavage patterns between ICSI and c-IVF when accounting for the expected fertilization delay in c-IVF.
- The findings suggest that the routine application of ICSI does not provide a developmental advantage to embryos and may actually reduce the pool of high-quality embryos available for transfer or vitrification.
Background and Clinical Context
The clinical rationale for choosing ICSI over c-IVF in the absence of male factor infertility is often centered on preventing the devastating clinical outcome of zero fertilization. However, randomized controlled trials have increasingly shown that for couples with normal or mildly impaired sperm parameters, ICSI does not improve live birth rates. While the clinical outcomes (pregnancy and live birth) are well-documented, the impact of the fertilization method on the biological ‘journey’ of the embryo—from the moment of sperm entry to the blastocyst stage—has been less clear.
The advent of Time-Lapse Monitoring (TLM) has allowed embryologists to observe embryo development in unprecedented detail, capturing discrete morphokinetic events that were previously missed during static observations. Understanding whether the mechanical bypass of natural fertilization in ICSI alters these developmental milestones is essential for optimizing laboratory protocols and patient outcomes.
Study Design and Methodology
The INVICSI study was an open-label, multicentre, randomized controlled trial (RCT) designed to compare ICSI and c-IVF in patients where severe male factor infertility was not present. Eligible participants were women with a male partner whose semen analysis showed at least 2 million progressively motile spermatozoa, as well as couples using donor sperm.
This secondary analysis included 824 patients recruited between November 2019 and December 2022. The primary focus was on embryo quality and destiny (utilization for transfer or vitrification) and morphokinetic parameters. A subgroup of 482 participants (247 in the ICSI group and 235 in the c-IVF group) provided time-lapse data from three specific trial sites. Embryos were evaluated based on Gardner’s criteria for blastocyst quality and monitored for specific cleavage patterns, including multinucleation at the two- or four-cell stage, direct cleavage, and reverse cleavage.
Key Findings: Embryo Quality and Utilization
The results of the analysis revealed several significant differences in embryo development between the two groups. While the number of retrieved oocytes was comparable, the ‘destiny’ of those oocytes differed markedly according to the fertilization method.
Lower Blastocyst Yield in the ICSI Group
Compared to c-IVF, ICSI was associated with fewer cleaved embryos on Day 2 and a lower total number of blastocysts. Crucially, the ICSI group produced fewer high-quality blastocysts on Day 5 based on Gardner’s criteria (P < 0.05). This translated to a lower number of blastocysts utilized for either fresh transfer or vitrification. Interestingly, while Day 5 quality was lower in the ICSI group, there were no significant differences in the number of high-quality blastocysts on Day 6, suggesting a potential catch-up effect or that the detrimental effects of ICSI are most visible at the standard Day 5 milestone.
Fertilization Abnormalities
ICSI did result in fewer abnormally fertilized embryos (specifically 1PN and >2PN embryos) compared to c-IVF. This is expected, as ICSI bypasses the natural mechanisms that can lead to polyspermy or failed syngamy in a dish. However, this reduction in abnormal fertilization did not translate into a higher yield of usable embryos; in fact, the opposite was true.
Key Findings: Morphokinetics and Cleavage Patterns
One of the most significant aspects of this study was the analysis of time-lapse data. Because ICSI involves the immediate introduction of sperm into the oocyte, whereas c-IVF allows for a window of time during which fertilization occurs naturally, researchers adjusted the c-IVF timing to account for this delay.
Developmental Synchronization
Once the fertilization timing was adjusted, there were no significant differences in the timing of embryo development between the two groups. The embryos reached the 2-cell, 4-cell, 8-cell, and blastocyst stages at roughly the same rates. Furthermore, the incidence of developmental ‘errors’—such as multinucleation, direct cleavage (where a cell divides into three instead of two), or reverse cleavage—did not differ significantly between the groups. This suggests that while ICSI might influence the overall quality and viability of the embryo pool, it does not fundamentally alter the speed or the structural pattern of early cellular division.
Expert Commentary
These findings add to a growing body of evidence that challenges the ‘ICSI-for-all’ approach. From a biological perspective, the lower yield of high-quality blastocysts in the ICSI group may be attributed to several factors. First, c-IVF allows for a degree of natural selection, where only the most ‘fit’ sperm successfully penetrate the zona pellucida and oolemma. Second, the mechanical trauma of the ICSI needle and the injection of a small amount of medium into the ooplasm may induce subtle stress that manifests as lower blastocyst quality by Day 5.
It is also worth noting the study’s limitations. The researchers used traditional sperm parameters (count and motility) to define the study population. They did not assess sperm DNA fragmentation (SDF) or other advanced markers. Some clinicians argue that ICSI may benefit patients with high SDF even if motility is normal, though this remains a point of debate in the field. Furthermore, while the study shows fewer high-quality blastocysts on Day 5, the blastocyst quality in the first embryo transfer did not differ between groups, which may explain why live birth rates often remain similar in large-scale trials.
Conclusion
The INVICSI secondary analysis provides compelling evidence that in patients without severe male factor infertility, ICSI does not offer a developmental advantage over conventional IVF. In fact, ICSI appears to result in a lower yield of high-quality Day 5 blastocysts. Given that c-IVF is less labor-intensive, more cost-effective, and respects the natural selection processes of fertilization, these findings support the recommendation that c-IVF should remain the preferred fertilization method for non-male factor cases. Clinicians should be cautious about routine ICSI use, as it may reduce the total number of high-quality embryos available for a patient’s treatment cycle.
Funding and Registration
The INVICSI study was supported by an unrestricted grant from Gedeon Richter and funded by the Capital Region of Denmark (A6606), Læge Sofus Carl Emil Friis og Hustru Olga Doris Friis’ Legat, and Amager/Hvidovre Hospital. The trial is registered under ClinicalTrials.gov (NCT04164108).
References
Berntsen S, Zedeler A, Grøndahl ML, et al. Early embryo developmental kinetics following IVF versus ICSI in patients without severe male factor infertility: a secondary analysis of a multicentre, randomized controlled trial (INVICSI). Hum Reprod. 2025;40(10):1877-1885. doi:10.1093/humrep/deae157.