How Eggs Are Fertilized in the Lab: ICSI vs. Conventional Insemination
Fertilization is one of the most crucial steps in the IVF process, where an egg and sperm unite to form an embryo. In the lab, embryologists use two primary techniques to achieve fertilization: Intracytoplasmic Sperm Injection (ICSI) and Conventional Insemination. Both methods aim to create healthy embryos, but they differ in their approach and when they are used.
Conventional Insemination: Letting Nature Take Its Course
The image is focused on the eggs, which are 3D, but the tiny sperm is in the background preparing for the journey of a lifetime
Conventional insemination is the more “natural” approach to fertilization in the lab. This method mimics what happens inside the body, as sperm are placed in a dish with the egg and left to fertilize on their own.
How It Works:
A carefully prepared sample of sperm is placed into a dish containing the retrieved eggs.
The sperm swim toward the egg, and one successful sperm penetrates its outer layer (zona pellucida) to fertilize it.
After about 16-18 hours, embryologists check for signs of fertilization, looking for the presence of two pronuclei (one from the egg and one from the sperm).
When is Conventional Insemination Used?
When the sperm sample is strong (good count, motility, and morphology).
When there are no known male-factor infertility issues.
When the egg’s outer shell is not too thick, allowing sperm to penetrate naturally.
While this method allows for a more natural selection of sperm, not all eggs fertilize successfully, especially if the sperm sample is weak or has trouble penetrating the egg. That’s where ICSI comes in.
ICSI: A More Precise Approach
Intracytoplasmic Sperm Injection (ICSI) is a more advanced fertilization technique used when sperm need extra help reaching and fertilizing the egg. Instead of relying on sperm to penetrate the egg on their own, embryologists select a single sperm and inject it directly into the egg using a fine glass needle under a high-powered microscope.
Moments before injection
How It Works:
The egg is denuded or stripped, meaning its outer cumulus cells are gently removed to allow for better visualization.
A single, healthy sperm is selected and immobilized.
The sperm is loaded into a fine needle and injected directly into the egg’s cytoplasm.
After the procedure, the eggs are placed in the incubator, and fertilization is checked the next day.
When is ICSI Used?
Male Factor Infertility – Low sperm count, poor motility, or abnormal sperm shape.
Previous Fertilization Failure – If eggs failed to fertilize using conventional insemination.
Frozen Eggs – Thawed eggs often have a harder outer shell, making ICSI more effective.
PGT (Genetic Testing) – If embryos will be biopsied, ICSI is preferred to avoid contamination from excess sperm.
Because ICSI bypasses the natural fertilization barriers, it significantly increases the chances of fertilization for couples facing male infertility challenges. However, like all procedures, it requires expertise and precision from the embryology team.
Which Method is Better?
Neither method is better—it depends on the patient’s situation. Conventional insemination allows for natural sperm selection, but ICSI ensures fertilization when sperm struggles to reach the egg. In many IVF cycles, both methods are used on different eggs from the same patient to maximize fertilization success.
There are plenty of studies out there that suggest one method may be better than others, but the percentage is usually not significant. A good embryologist will assess the patient’s history and specimens before selecting a method. Sometimes, patients will do their own research and ask the lab to fertilize with their preferred method. This isn’t recommended as the embryologist will have a better assessment!
Regardless of the method used, fertilization is just the beginning. The journey continues as fertilized eggs grow into embryos, are monitored for development, and are eventually transferred back into the uterus in hopes of a successful pregnancy.
“Whether we let the sperm swim or give it a VIP injection with ICSI, every egg gets a recipe for success—because in this lab, we don’t just crack eggs, we make miracles!”
