During conventional IVF, fertilization depends on sperm reaching and penetrating the egg on their own.
When that doesn’t happen, even healthy eggs may never begin developing into embryos. Intracytoplasmic sperm injection (ICSI) was developed to help fertilization occur in situations where it otherwise might not.
This article covers what it is, how it works, who it’s actually for, and what it can and can’t improve in an IVF cycle.
Key Takeaways
What Is ICSI?
ICSI, intracytoplasmic sperm injection (pronounced “ick-see”), is a lab technique used during IVF. An embryologist injects a single sperm directly into an egg instead of letting fertilization happen on its own in a dish. Everything else in the cycle stays the same. Stimulation medications, monitoring, and egg retrieval all proceed exactly as they would without ICSI. The difference happens entirely in the lab, on the day eggs and sperm are combined.
How Does ICSI Work?
The embryologist works under a high-powered microscope fitted with a micromanipulator, a tool that allows extremely small, precise movements. Two fine glass tools do the work together: one holds the egg steady, the other delivers a single sperm directly into it.
Once the sperm is inside, the embryologist withdraws the pipette, and the egg is left to fertilize on its own from that point forward.
Not every retrieved egg is ready for this step. Only mature eggs, identified by a specific stage of cellular development, can be injected. Immature eggs are set aside, since injecting them would not lead to normal fertilization.
Egg maturity is one of the factors the lab assesses on the day of retrieval, before deciding how many eggs are suitable for ICSI. What happens next follows the same general timeline as any IVF cycle, broken down day by day:
What Happens After Injection
Day 1
Fertilization is confirmed. The embryologist checks whether the injected egg has fertilized normally.
Day 3
The embryo reaches the cleavage stage. Fertilized eggs divide into multiple cells as they continue developing.
Day 5
The embryo reaches the blastocyst stage. This is the stage most embryos are transferred or frozen at, for those still developing.
If PGT is planned
Genetic testing is performed. A small number of cells are taken from the embryo at the blastocyst stage, before it’s frozen.
Next Step
The embryo is transferred or frozen. Depending on the plan for that cycle, it’s moved to the uterus or frozen for a future transfer.
How Long Does ICSI Take?
The injection itself takes only a few minutes per egg. It’s performed on the same day as egg retrieval, once the eggs have been assessed for maturity. Fertilization is checked the following day, so patients typically get a fertilization update within about 24 hours of retrieval.
A reproductive endocrinologist oversees the cycle as a whole, but the injection itself is performed by a trained embryologist in the IVF lab.
ICSI vs. Conventional IVF Fertilization
Conventional insemination and ICSI aren’t alternatives a patient chooses between. Both are fertilization methods used within the same IVF cycle, and which one a lab uses depends on the sperm sample and cycle history, not preference.
Some cycles use ICSI for every retrieved egg. Others use conventional insemination for most eggs and reserve assisted fertilization for cases where the sperm sample warrants it. The decision is made in the lab, based on that day’s sample.
Who May Benefit From ICSI?
Male-Factor Infertility
- Sperm count is low or absent, or a high percentage of sperm are abnormally shaped, a pattern often described as severe male infertility
- Sperm can’t be ejaculated but can be surgically collected from the testicles or epididymis, often due to an irreversible vasectomy or an injury
- Erection or ejaculation is affected by a spinal cord injury, diabetes, or another underlying health condition
Fertilization Concerns
- Sperm can’t reach the egg at all, or reaches the egg but fails to fertilize it on its own
- A prior IVF cycle had failed or unexpectedly low fertilization with conventional insemination
Laboratory Situations
- Frozen eggs are being used, since freezing can affect the egg’s outer layer and make natural sperm penetration less reliable
- Genetic testing (specifically PGT-M) is planned on the resulting embryos, since a single confirmed sperm source reduces the risk of extra sperm cells contaminating the sample
A physician at RSMC’s San Diego fertility center reviews the sperm analysis and cycle history to determine whether ICSI fits a specific IVF plan.
Why Isn’t ICSI Used for Everyone?
ICSI requires extra embryology work for every egg it’s used on. It’s built to solve a specific problem, not to improve every cycle by default. Conventional insemination already works well for most patients with normal semen parameters. Adding ICSI to a cycle that doesn’t need it means more lab handling without a clear benefit. The technology being available isn’t itself a reason to use it.
The American Society for Reproductive Medicine’s (ASRM) 2026 clinical guidance reaches the same conclusion. In patients without male-factor infertility, routine ICSI use does not improve live birth rates compared with conventional insemination. Clinics generally reserve ICSI for cycles with a specific indication, such as male-factor infertility or a prior fertilization failure. It isn’t applied universally.
How Fertility Specialists Decide Whether to Use ICSI
Deciding whether to use ICSI isn’t based on a single factor. A fertility specialist typically reviews:
- The semen analysis in detail
- Any fertilization history from a prior cycle
- Whether genetic testing is planned for the resulting embryos
- Egg quantity and maturity from that specific retrieval, since the lab needs enough mature eggs available to make the additional handling worthwhile
In many cases, the final call isn’t made until the day of retrieval. That’s when the actual sperm sample and egg maturity are known, rather than decided in advance.
Success Rates: What ICSI Can and Cannot Improve
“Success” isn’t one number. A full IVF cycle passes through several distinct stages, and ICSI’s effect is concentrated in just the first one:
A Full IVF Cycle Has Six Distinct Stages
Sperm injection success — whether the injection itself was completed without damaging the egg
Fertilization rate — whether the injected egg fertilizes and begins dividing
Blastocyst development — whether the fertilized egg develops into a viable embryo several days later
Implantation — whether a transferred embryo attaches to the uterine lining
Clinical pregnancy — whether a pregnancy is confirmed
Live birth — whether the pregnancy results in a birth
Bar length reflects how directly ICSI influences each stage, not a statistical probability.
Fertilization rate with ICSI varies considerably depending on why it’s being used. ASRM’s clinical guidance cautions against treating fertilization rate as a stand-in for the outcome that actually matters, since a higher fertilization rate doesn’t reliably translate into a higher live birth rate. No single percentage applies evenly across every reason ICSI might be used.
ICSI Improves Fertilization, Not Every Aspect of IVF
Patients sometimes assume that because one sperm is directly injected, pregnancy is close to guaranteed. It isn’t. ICSI solves one specific problem: helping a sperm enter an egg. It doesn’t improve:
- Embryo quality
- Chromosomal abnormalities
- Implantation odds
- Miscarriage risk
- Uterine factors
- The effect of maternal age on egg quality
Those factors depend on the egg and embryo themselves, and on the uterine environment, not on how fertilization happened.
What Are the Risks and Limitations of ICSI?
ICSI carries the same general risks as any IVF lab procedure. A small percentage of eggs may be damaged during injection, and not every injected egg fertilizes.
According to the ASRM Committee Opinion, current evidence has not shown a significant increase in congenital anomalies attributable to ICSI itself. This includes recent randomized and cohort studies of appropriately selected patients without severe male-factor infertility.
Researchers continue to study whether previously observed differences relate more to the underlying causes of infertility than to the procedure itself. This is worth discussing directly with a physician who can speak to how current guidance applies to a specific situation.
Common Myths About ICSI
Myth
“ICSI guarantees fertilization.”
Reality
It increases the likelihood of fertilization in cases where it’s needed, but some injected eggs still won’t fertilize.
Myth
“ICSI is better than conventional IVF.”
Reality
Neither is universally better. The right method depends on the specific infertility diagnosis, not on which technique sounds more advanced. Current clinical guidance specifically advises against routine ICSI use when there’s no male-factor indication.
Myth
“Every IVF patient needs ICSI.”
Reality
Many IVF cycles achieve strong fertilization with conventional insemination alone, and ICSI is reserved for cycles with a specific indication.
Myth
“ICSI always produces healthier embryos.”
Reality
ICSI affects fertilization, not embryo quality. An embryo’s development depends on the egg and sperm’s underlying genetic and cellular quality, not on how fertilization occurred.
Frequently Asked Questions
Is ICSI the same as ICI?+
Does ICSI guarantee fertilization?+
Can an egg be damaged during ICSI?+
Is ICSI recommended for every IVF cycle?+
Does ICSI increase the risk of birth defects?+
Can frozen sperm be used for ICSI?+
Can poor-quality sperm still be used for ICSI?+
What happens if fertilization doesn’t occur after ICSI?+
Wondering whether ICSI is right for your IVF cycle?
Talk with an RSMC fertility specialist in San Diego to review your specific situation.
Table of Contents
- What Is ICSI?
- How Does ICSI Work?
- ICSI vs. Conventional IVF Fertilization
- Who May Benefit From ICSI?
- Why Isn’t ICSI Used for Everyone?
- How Fertility Specialists Decide Whether to Use ICSI
- Success Rates: What ICSI Can and Cannot Improve
- What Are the Risks and Limitations of ICSI?
- Common Myths About ICSI
- Frequently Asked Questions
