Inside the Embryo Lab: What Happens to Your Eggs After Retrieval

For many patients going through IVF, the egg retrieval is a major milestone—but it's also the start of a fascinating and delicate process that unfolds largely behind the scenes. What happens next takes place inside the embryology lab, where highly trained scientists guide the earliest stages of life with precision, care, and advanced technology. 

So what exactly happens to your eggs after retrieval? Let’s take a closer look at what goes on inside the embryo lab. 

Day 0: Fertilization 

Once your eggs are retrieved, they’re immediately taken to the lab (the doctor will hand them to the embryologist directly during the retrieval) and assessed for maturity. Mature eggs are then fertilized with sperm—either by mixing them together (conventional IVF) or by injecting a single sperm directly into each egg (a process called ICSI, or intracytoplasmic sperm injection). The goal is to create viable zygotes that will begin dividing into embryos. 

Day 1–3: Early Cell Division 

By the next day (Day 1), lab staff check to confirm successful fertilization. A fertilized egg will typically show two pronuclei (PNs)—one from the egg and one from the sperm. If these are present, the embryo is considered normally fertilized. 

Over the next few days (Days 2–3), embryos begin to divide, forming 2-cell, then 4-cell, then 8-cell structures. Embryologists closely monitor this development, assessing not just the number of cells, but how evenly they divide, whether there is fragmentation, and how the embryo is growing in comparison to others in the same cohort. For some patients at CHR we will choose to transfer on day 3. To learn more about the reasons why listen to our podcast episode on Day 3 vs. Day 5 transfer on our podcast The Egg Factor.  

Day 5–6: Blastocyst Development 

Around Day 5 or 6, healthy embryos that continue to develop reach the blastocyst stage. A blastocyst is a fluid-filled structure made up of hundreds of cells that have begun to specialize. At this stage, embryologists evaluate each embryo’s morphology—its size, expansion, and cellular structure—before selecting the most viable ones for transfer or freezing. 

Fresh Transfer or Freezing? 

Depending on your protocol, embryos may be transferred into the uterus as part of a fresh transfer, or they may be frozen (vitrified) for a later cycle. Some patients may also choose to send biopsies of embryos for genetic testing (such as PGT-A), which requires the collection of a few cells before freezing the embryo. It’s important to note that CHR has long been critical of overusing genetic testing, especially in older women or those with low embryo numbers, due to the risk of discarding embryos that might still be viable. 

Why Lab Quality Matters 

Embryo development is incredibly sensitive to environmental conditions, including temperature, air quality, and handling. That’s why CHR's embryology lab is designed with rigorous controls and staffed by experienced embryologists who specialize in treating complex and poor-prognosis cases. From fertilization through embryo transfer, each step is carefully tailored to support your specific fertility needs. 

Final Thoughts 

While the embryo lab may be out of sight, it’s never out of mind. Behind each microscope and incubator is a team committed to giving your embryos the best possible chance at success. Understanding what happens after retrieval can offer reassurance—and help you feel more connected to the process as your journey continues.