What Happens to Embryos in the Lab: Day 1 to Day 5 Explained

If you're going through IVF, one of the most common questions is: "What's actually happening to my embryos in the lab?"

While much of the process happens behind the scenes, the days between fertilization and embryo transfer are critical. Understanding this journey—from day 1 to day 5—can help you feel more informed and confident during treatment.

The IVF Lab: Where Development Begins

After egg retrieval and fertilization, embryos are carefully cultured in a highly controlled laboratory environment designed to mimic the conditions of the body.

Embryologists monitor development closely, evaluating how embryos grow and divide over time.

Day 1: Fertilization Check

About 16–18 hours after fertilization, embryologists check to see which eggs have successfully fertilized.

At this stage, a normally fertilized egg will show:

• Two pronuclei (one from the egg, one from the sperm)
• Early signs of cell division

Not all eggs fertilize—and that's completely normal.

Day 2: Early Cell Division

By day 2, embryos typically divide into 2–4 cells.

Embryologists assess:

• Number of cells
• Symmetry of the cells
• Presence of fragmentation (cell debris)

Steady, even division is a positive sign—but variation is common.

Day 3: Cleavage Stage

By day 3, embryos usually reach 6–10 cells.

This stage is known as the cleavage stage, where rapid cell division continues.

At this point, some clinics may:

• Consider early transfer (less common today)
• Continue growing embryos to day 5 for better selection

Day 4: Morula Stage

On day 4, embryos form a structure called a morula—a tightly packed ball of cells.

At this stage:

• Individual cells become less distinct
• The embryo begins preparing for more complex development

Not all embryos reach this stage at the same pace, and that's normal.

Day 5: Blastocyst Stage

By day 5, some embryos develop into blastocysts, which is the stage most commonly used for transfer or freezing.

A blastocyst has:

• An inner cell mass (future baby)
• An outer layer (future placenta)
• A fluid-filled cavity

This is a key milestone because:

• Embryos that reach this stage are more likely to implant
• It allows for better selection of viable embryos

Why Not All Embryos Make It to Day 5

It's important to understand that not every embryo will continue developing.

This is a natural selection process:

• Some embryos stop growing due to genetic or developmental issues
• Others progress at different rates

While it can be difficult emotionally, this process helps identify the embryos with the highest potential.

What Happens After Day 5?

Once embryos reach the blastocyst stage, they may be:

Transferred

A selected embryo is placed into the uterus during an embryo transfer procedure.

Frozen (Cryopreserved)

High-quality embryos can be frozen for future use.

Biopsied for Genetic Testing (PGT-A)

In some cases, a few cells are removed from the embryo to assess chromosomal health before transfer.

How Embryologists Choose the Best Embryo

Embryo selection is based on multiple factors:

• Development speed (day 5 vs. day 6)
• Cell structure and appearance (grading)
• Genetic testing results (if performed)

This helps optimize the chances of implantation and a healthy pregnancy.

Why the Lab Environment Matters

The IVF lab plays a critical role in success.

Key factors include:

• Air quality and temperature control
• Culture media used to support embryos
• Embryologist expertise
• Laboratory technology

Even small differences in lab conditions can impact embryo development.

Common Patient Questions

"Should I be worried if not all embryos make it?"
No—this is expected. It's part of the natural selection process.

"Is it better to transfer on day 3 or day 5?"
Day 5 (blastocyst transfer) is more common today because it allows for better embryo selection.

"What if none of my embryos reach day 5?"
Your doctor may adjust your protocol or discuss alternative strategies for future cycles.

Final Thoughts

The journey from fertilization to blastocyst is a delicate and complex process.

While much of it happens in the lab, every step is carefully monitored to give your embryos the best possible chance of success.

Understanding what happens during these critical days can help you feel more informed—and more in control—throughout your IVF journey.