ICSI, or intracytoplasmic sperm injection, is a fertilization technique used during IVF where an embryologist injects a single sperm directly into an egg. In a standard IVF cycle, eggs and sperm are placed together in a dish and left to fertilize on their own. With ICSI, that step is replaced by precise micromanipulation under a microscope, bypassing many of the natural barriers sperm normally need to overcome. ICSI is now the most common fertilization method in IVF, used in roughly 75% of cycles with a patient’s own eggs.
How ICSI Works
The IVF process leading up to ICSI is identical to conventional IVF: ovarian stimulation, egg retrieval, and preparation in the lab. The difference is entirely in how fertilization happens. Once mature eggs are collected, an embryologist uses a fine glass holding pipette to stabilize a single egg. A second, much thinner needle is used to pick up one sperm that has been immobilized (so it won’t damage the egg’s interior). The needle pierces the outer shell and membrane of the egg, reaches the cytoplasm (the fluid interior), and deposits the sperm. The needle is then withdrawn.
Each mature egg gets its own individually selected sperm. The process takes only a few minutes per egg, though the preparation and selection steps add time. After injection, the eggs are monitored for signs of normal fertilization over the next 16 to 18 hours. From that point forward, embryo culture, grading, and transfer proceed exactly as they would in conventional IVF.
About 5% to 10% of eggs don’t survive the injection process. The benchmark most labs aim for is a damage rate below 10%. This is one reason clinics prefer to have multiple mature eggs to work with.
When ICSI Is Recommended
ICSI was originally developed for severe male factor infertility, and that remains its strongest use case. It’s recommended when sperm counts are very low (under 10 to 20 million per milliliter), when motility is poor (under 30% to 50% of sperm moving), or when sperm shape is significantly abnormal. It’s also the only option when sperm must be surgically retrieved from the testicle or epididymis, which is the case for men who produce no sperm in their ejaculate due to a blockage or severely impaired sperm production.
For men with borderline semen parameters (counts between 10 and 20 million, motility between 30% and 50%), studies show ICSI produces significantly better fertilization rates than conventional insemination. For men with normal semen, however, fertilization rates and pregnancy rates are similar between the two methods.
ICSI for Non-Male Factor Infertility
Despite being designed for male infertility, ICSI is now frequently used in cases of unexplained infertility, low egg yield, and as a general safeguard against fertilization failure. The evidence here is more mixed.
For unexplained infertility, ICSI does increase the fertilization rate per egg by about 30% compared to conventional insemination. More notably, fertilization failure (where zero eggs fertilize) is over eight times more likely with conventional insemination than with ICSI. That’s a meaningful safety net. But the catch is that ICSI for unexplained infertility has not been shown to improve live birth rates. More eggs fertilize, yet the number of babies born is statistically the same.
For women with a low number of retrieved eggs, the picture is similar. A controlled trial of patients with six or fewer eggs found nearly identical outcomes between ICSI and conventional insemination: fertilization rates of 77.7% versus 70.2%, identical fertilization failure rates, and comparable pregnancy and miscarriage rates. The American Society for Reproductive Medicine’s analysis of national registry data found that in women with diminished ovarian reserve, ICSI was actually associated with a slightly lower live birth rate (20.4% versus 21.9%) compared to conventional IVF.
So why is ICSI used so widely? Largely to avoid total fertilization failure, which can mean a canceled cycle and significant emotional and financial cost. Even when it doesn’t improve birth rates, eliminating the risk of zero fertilization is valuable to many patients and clinics.
Fertilization Rates: ICSI vs. Conventional IVF
When researchers split a patient’s eggs between the two techniques in the same cycle, ICSI fertilizes a higher percentage of the eggs that are actually injected: about 72% compared to 59% for conventional IVF. Total fertilization failure occurred in 4% of conventional IVF attempts versus 1.4% of ICSI attempts in one large split-cycle study. In another study of patients switching from intrauterine insemination to IVF, total fertilization failure happened in 9% of conventionally inseminated egg batches and 0% of ICSI batches.
ICSI also produces fewer abnormally fertilized eggs (those with too many or too few pronuclei), which means a cleaner batch of embryos to work with. The rate of abnormal fertilization with three pronuclei, for example, was 3% with ICSI compared to 7% with conventional insemination.
Live Birth Rates After ICSI
Live birth rates depend far more on maternal age and ovarian reserve than on whether ICSI or conventional insemination was used. Based on U.S. national surveillance data from 2016 to 2018, a 35-year-old patient with unexplained infertility, a BMI of 25, and no prior pregnancies has roughly a 48% chance of a live birth after one egg retrieval cycle, 69% after two, and 80% after three. Those numbers drop significantly with age or diminished ovarian reserve: a 40-year-old with unexplained infertility has about a 25% chance after one retrieval and 52% after three.
These figures reflect the overall IVF population, the vast majority of whom used ICSI. The technique itself doesn’t appear to change live birth odds compared to conventional IVF when the underlying cause of infertility is not sperm-related.
Risks Specific to ICSI
The major concern with ICSI is a small but measurable increase in birth defects. The American Society for Reproductive Medicine has reported a 4.2% risk of major congenital malformations across IVF and ICSI cycles combined. When the two are separated, ICSI carries a 9.9% risk of birth defects compared to 7.2% for conventional IVF. Compared to unassisted conception, ICSI was associated with a 77% higher likelihood of a birth defect, while conventional IVF was 26% higher. After adjusting for other factors like parental age and underlying infertility diagnoses, the increased risk with conventional IVF lost statistical significance, but the increased risk with ICSI remained.
The most commonly documented defects are musculoskeletal, cardiovascular, and urogenital abnormalities. It’s worth noting that couples using ICSI often have more severe underlying fertility issues, which may independently contribute to these risks.
There are also theoretical concerns about epigenetic effects. Because ICSI bypasses normal sperm selection and involves physical manipulation of the egg during a sensitive period of genetic reprogramming, some researchers have flagged a possible link to rare imprinting disorders like Beckwith-Wiedemann syndrome (about 1 in 100,000 births) and Angelman syndrome (about 1 in 300,000). The absolute numbers are very small, and other researchers have argued the association is not clearly established.
Why ICSI Is Used So Often
The shift toward near-universal ICSI use is driven more by risk management than by evidence of better outcomes. For clinics, total fertilization failure is a worst-case scenario: the patient has gone through stimulation, retrieval, and significant expense, and ends up with no embryos. ICSI virtually eliminates that possibility. For patients with male factor infertility, it’s clearly the right choice. For everyone else, the decision involves weighing a modest reduction in fertilization failure risk against slightly higher costs and a small increase in birth defect risk. Many clinics default to ICSI for all cycles, while others reserve it for cases where it’s most likely to make a difference.