Understanding a Low Fertilization Rate

What Does a Low Fertilization Rate Mean?

During IVF, fertilization rate describes the proportion of mature eggs that fertilize normally after exposure to sperm through conventional insemination or after sperm is injected directly into an egg using ICSI.

A low fertilization rate means fewer mature eggs fertilized normally than expected. Because fertilization depends on both egg and sperm biology as well as the number of mature eggs available, the result does not necessarily identify one specific cause.

What Can Affect Fertilization During IVF?

Several factors can influence fertilization:

• Egg maturity: Eggs generally need to reach the appropriate stage of maturity to fertilize normally.
• Egg factors: Biological characteristics of the egg can influence whether fertilization occurs.
• Sperm factors: Sperm concentration, motility and other characteristics may affect conventional fertilization, while some sperm-related factors can remain relevant even when ICSI is used.
• Fertilization method: Conventional insemination and ICSI use different approaches to bring sperm and egg together.
• Biological variation: Fertilization rates can vary between patients and between treatment cycles.

Your physician and embryology team can review the number of mature eggs, sperm findings and laboratory results together to understand what the fertilization rate may mean for future treatment.

What Your IVF Cycle Results Can Tell Us

When fertilization is lower than anticipated, reviewing the entire IVF cycle can help determine where attrition occurred and whether a meaningful pattern emerged. The number of follicles that developed, eggs retrieved, mature eggs available for fertilization and normally fertilized eggs each provide different information.

For example, retrieving several eggs but having relatively few mature eggs presents a different question from having many mature eggs but relatively few fertilize. Similarly, a fertilization issue is distinct from a cycle in which fertilization is within the expected range but relatively few embryos subsequently develop to the blastocyst stage.

The fertilization method can also be relevant. With conventional insemination, eggs and sperm are placed together in the laboratory, while ICSI involves an embryologist injecting a single sperm directly into a mature egg. ICSI can address certain barriers to fertilization, but it cannot eliminate all egg-, sperm- or embryo-related factors that may affect development.

Your Spring Fertility physician and embryology team can review these laboratory details together. Understanding precisely what happened at each stage can provide more actionable information than the fertilization percentage alone and may help inform decisions about a future IVF cycle.

What Happens After a Low Fertilization Rate?

After a low fertilization rate, your physician and embryology team can review the laboratory results to understand exactly where attrition occurred. The number of eggs retrieved, number of mature eggs, sperm findings and fertilization method can all provide important context.

If relatively few eggs were mature, the review may focus partly on egg maturity and ovarian stimulation. If a substantial number of mature eggs were available but relatively few fertilized normally, your team may consider egg factors, sperm factors and how fertilization was performed. Conventional insemination and ICSI use different laboratory approaches, so the fertilization method is also relevant when evaluating the result.

A single IVF cycle does not always establish why fertilization was lower than expected, and not every contributing biological factor can be identified or changed. However, the information gained from the cycle can still be useful when considering another attempt.

Your Spring Fertility physician and embryology team can review the complete cycle and determine whether any changes to stimulation, fertilization strategy or other aspects of treatment should be considered in a future IVF cycle.