Embryo development in IVF is an incredible process, combining the precision of science with the hope of creating new life.

You might wonder exactly how your embryos form and grow and what happens during the earliest stages of life.

In this blog, we’re breaking down the day-by-day embryo development phases. From fertilization to blastocyst formation, you’ll discover what makes these stages critical in setting the foundation for a healthy pregnancy.

But first, let’s explore how monitoring embryo development helps embryologists make the best decisions for your IVF journey.

Why Embryo Development in IVF Matters

While the biological phases of embryo development in IVF are the same as in natural conception, the controlled environment of IVF offers a unique advantage. It allows embryologists to monitor progress closely, assess key factors like cell division and structural formation, and determine each embryo’s grade. 

By observing milestones in embryo development, embryologists gain valuable insights into an embryo’s health and viability. This information guides critical decisions about which embryos to transfer or freeze, increasing the likelihood of a successful pregnancy.

Now, let’s explore the stages of embryo development to understand what happens within the delicate environment of your embryologist’s petri dish. 

Key Stages of IVF Embryo Development

Day 0: Fertilization

What Happens: In IVF, fertilization occurs in a laboratory when an embryologist combines an egg with sperm under carefully controlled conditions. This process results in a single-cell zygote, marking the start of embryo development.

Why It Matters: Embryologists monitor this stage to confirm zygote formation and rule out abnormalities that might prevent further development. After fertilization, the zygote starts to develop quickly, with significant changes happening over the next few days.

Day 1: Zygote Division

What Happens: 24 hours after fertilization, the single-cell zygote undergoes its first division. It splits or “cleaves” into two identical cells. At this stage, the zygote is now considered an embryo. 

Why It Matters: The first division is a critical milestone. Embryologists confirm proper cell division for the embryo’s viability. Any abnormalities during this phase can impact the embryo’s ability to develop further.

Day 2: Early Cleavage Stage

What Happens: Each of the embryo’s cells divides into two, resulting in a total of four cells. This marks an important phase in the cleavage stage, where the embryo begins forming its early structure. 

Why It Matters: Embryologists monitor the rate and quality of cell division to ensure the embryo is progressing normally. If it hasn’t divided by this point, it is considered non-viable.

They also look for fragmentation, which refers to small pieces of cytoplasm that break off during cell division. Minimal fragmentation indicates a healthy embryo, while excessive fragmentation can reduce the embryo’s potential for further development.

Day 3: Cleavage Stage and Embryo Grading

What Happens: The embryo divides further and typically consists of six to eight cells, marking the later phase of the cleavage stage. At this point, embryologists assess key characteristics to determine the embryo’s grade, including cell number, cell symmetry, and fragmentation. Some clinics even use artificial intelligence to refine grading precision.

An embryo transfer could take place on this day if your embryologist determines the embryo is ready for implantation.

Why It Matters: Grading on day three is essential to monitoring embryo development in IVF. It helps embryologists determine whether an embryo is ready for transfer or freezing, or if it needs more time to develop. 

Day 4: Morula Stage

What Happens: The embryo forms a compact ball of approximately 16-32 tightly packed cells, known as a morula. This process, called compaction, is critical as it prepares the embryo for the next stage of development.

Why It Matters: Successful compaction indicates the cells are communicating and organizing properly, setting the stage for blastocyst formation.

Day 5: Blastocyst Formation and Embryo Grading

What Happens: By day 5, the embryo develops into a blastocyst, containing around 70-100 cells and forming two key structures:

• Inner Cell Mass (ICM): A group of cells that develops into the fetus.
• Trophectoderm (TE): This outer layer becomes the placenta, providing nourishment to the embryo.

Embryologists evaluate the degree of embryo expansion along with the quality of the ICM and TE to assign a new grade. Fully expanded blastocysts increase the likelihood of implantation and are often selected for transfer on this day. Any embryos not yet fully expanded may continue to grow in the lab for one more day.

Some embryos also begin to hatch at this stage, separating from the surrounding shell called the zona pellucida. This hatching signals their readiness for implantation and often indicates a higher chance of success. However, some embryos may not hatch until after transfer.

Why It Matters: Day five represents a critical turning point as embryologists determine which embryos are ready for transfer or freezing, setting the stage for the next steps in your IVF journey.

Day 6: Final Day to Transfer or Freeze 

What Happens: Fully expanded embryos are re-evaluated to confirm if they’re ready for transfer or cryopreservation.

Why It Matters: Day six marks the final opportunity to preserve viable embryos. They must be either transferred or frozen at this stage, ensuring only those with the highest potential for a successful pregnancy are preserved.

During these six days of embryo development in IVF, embryos are carefully nurtured and monitored in the lab to ensure their best chance of success. After transfer, if the embryo successfully implants in the uterine lining, it will continue its journey toward a healthy pregnancy.

What if You Need to Transport Cryopreserved Embryos?

Your embryos are the result of incredible love, care, and precision during these early stages of development.

Moving your cryopreserved embryos might feel scary, but with the right transport method, you can rest easy knowing your cells are safe every step of the way. Whether they’re going to a different fertility clinic, another country for surrogacy, or a cryobank for storage, preserving their viability is crucial.

At CryoLogistics, we specialize in hand-carry cryogenic transport of reproductive cells. Our expert medical couriers personally oversee every detail of your transport and accompany your embryos from collection to delivery.

We ensure your precious cargo is always protected, maintained in optimal conditions for IVF, and safely delivered in just 24 to 72 hours.

We recognize the hopes and dreams invested in your irreplaceable embryos – they deserve to be transported in the safest hands possible. As you take this next step on your path to parenthood, we’d be honored to support you with this transition.

To learn more about our hand-carry cryo transport services for embryos, visit our website or chat with our team today.