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Fertilization of Eggs
One of the most preferred treatments for couples experiencing infertility is the in vitro fertilization (IVF) technique. In this treatment, the sperm and egg cells are brought together outside the body. In conventional IVF and advanced intracytoplasmic sperm injection (ICSI) applications, two main steps are performed: fertilization and embryo transfer [1].
Fertilization is the process in which the male reproductive cell, the sperm, and the female reproductive cell, the egg, combine to form the zygote that will enable the development of a baby. In IVF treatment, after ovarian stimulation, the retrieved eggs are evaluated in terms of maturity; only mature eggs are processed [1].
How Does the Fertilization Process Take Place?
The egg cells retrieved from the prospective mother and the sperm cells obtained from the male are fertilized under laboratory conditions. This stage applies to both conventional IVF and the microinjection method [1, 2]:
Conventional IVF: The selected egg cells are kept in special media for an average of 4 hours. Then, they are brought together with the sperm cells collected on the same day, and the sperm are expected to fertilize the egg naturally. Fertilization is evaluated under a microscope within 12–15 hours.
Microinjection (ICSI): A single sperm cell is directly injected into the egg, and the fertilization process is completed. ICSI is preferred instead of conventional IVF in cases where sperm motility is insufficient or when the number of eggs is 5 or fewer [1].
After fertilization, the eggs are stored in special incubators that mimic the temperature, humidity, and gas ratios of the body’s internal environment. The development of the zygotes is monitored under a microscope, and the fertilization status is examined [2].
For detailed information about the egg retrieval procedure, you can review our egg retrieval procedure page.
Why Does the Egg Fail to Fertilize?
In the majority of patients who do not achieve fertilization after microinjection, there are either severe problems with sperm parameters or poor egg quality. Total fertilization failure (TFF) after ICSI occurs in approximately 1–3% of couples receiving treatment [3]. These situations include the following [1, 3]:
- Performing ICSI with completely immotile sperm: Sperm motility is a critical indicator of fertilization capacity. Completely immotile sperm must be selected after viability testing.
- Acrosome deficiency or absence (globozoospermia): A deficiency or absence of the acrosome, the structure located in the head of the sperm, prevents penetration into the egg and activation.
- Unexplained fertilization failure: In some patients, fertilization may not occur even though sperm evaluation is normal and ovarian reserve is good; in such cases, oocyte activation defects should be assessed.
- Poor egg quality: Advanced age, diminished ovarian reserve, or a poor response to ovarian stimulation may negatively affect egg quality and reduce fertilization success.
In couples who experience fertilization failure, a comprehensive evaluation is performed before the next cycle, and the protocol is reorganized. If necessary, additional methods such as sperm activation techniques (calcium ionophore) or assisted oocyte activation may be applied [3].
For detailed information about embryo transfer, you can review our embryo transfer page.
Frequently Asked Questions (FAQ)
What is the difference between conventional IVF and microinjection (ICSI)?
In conventional IVF, the sperm and egg are brought together in the same environment, and natural fertilization is expected. In ICSI, a single sperm cell is directly injected into the egg. ICSI is especially preferred in cases of male-factor infertility involving low sperm count, poor motility, or morphology problems, and when the number of eggs is 5 or fewer.
When does fertilization occur and how is it determined?
Approximately 12–15 hours after the fertilization procedure is completed, the zygotes are examined under a microscope. The presence of two pronuclei (2PN) indicates successful fertilization. The embryologist informs the couple of the fertilization result, and embryo development is monitored daily.
Are all eggs fertilized?
No. Only mature eggs (at the MII stage) are used for fertilization. Not all mature eggs are capable of being fertilized; the average fertilization rate is reported to be approximately 70–80%. Among the fertilized embryos, the highest-quality ones are selected for transfer.
What should be done if fertilization fails?
When fertilization failure occurs, a comprehensive evaluation is performed before the next treatment. In the presence of acrosome deficiency or immotile sperm, oocyte activation techniques such as calcium ionophore may be used. If the issue is related to egg quality, the stimulation protocol is reorganized.
Does fertilization failure affect the next IVF attempt?
Each fertilization failure provides important information for the next cycle. Depending on the cause of failure, the protocol, medication dosage, or fertilization method may be changed to increase the chance of success. The majority of couples who experience total fertilization failure achieve fertilization in the following cycle.
References
- Showell, M. G., et al. (2023). In Vitro Fertilization. StatPearls — NCBI Bookshelf. NBK562266. (https://www.ncbi.nlm.nih.gov/books/NBK562266/)
- American Society for Reproductive Medicine (ASRM). In Vitro Fertilization Treatment Journey. ReproductiveFacts.org. (https://www.reproductivefacts.org/patient-journeys/in-vitro-fertilization-treatment/)
- Chatzimeletiou, K., et al. (2022). Total Fertilization Failure After ICSI: Causes and Management. Journal of Clinical Medicine. PubMed PMID: 35893577. (https://pubmed.ncbi.nlm.nih.gov/35893577/)
- Mouanness, M., et al. (2020). Cumulative Live-Birth Rates by Maternal Age after One or Multiple In Vitro Fertilization Cycles. PMC7139227. (https://pmc.ncbi.nlm.nih.gov/articles/PMC7139227/)