Embryo Development in IVF: What Happens in the First 5 Days?

The embryo development is one of the significant and intriguing aspects of the IVF journey. Understanding the process that takes place during the initial days will help couples feel more secure and confident.

“From a single fertilized egg to a growing embryo- every day in IVF tells an important story.”

In Origyn IVF, patients are often interested in knowing what happens in the IVF lab after fertilization and retrieval of eggs. How do embryos develop each day? What are the things that doctors and embryologists do? What do embryo grades mean?

In this blog, we’ll describe the entire procedure of embryo development in the initial four weeks of IVF in a simple manner. We will also learn about embryo stages, grading, cell division and the factors that make an embryo healthy.

Once the process of removing eggs is completed in IVF the eggs are fertilized with sperm in the lab. This is the point at which embryo development starts.

The following day, embryologists closely examine fertilized eggs under microscope to see if fertilization has happened successfully.

Healthy fertilized eggs look like a round shape with two small circles within it. The two pronucleus are:

• Male pronucleus
• Female pronucleus

Together, they suggest that fertilization is successful and that the embryo-formation process has started. This is the first indication of embryo development. 

On the first day, embryologists test to see if there is a presence of 2 pronuclei (2PN stage). These two pronuclei represent:

• One genetic component that comes from sperm.
• One genetic link comes from the egg

If there are two pronuclei that appear, this signifies that fertilization has taken place properly.

Key Features of Day 1 Embryos

At this time, the embryo remains one cell, however the process of embryo is officially started.

On Day 2, the fertilized egg begins to divide into a variety of cells. Usually, embryologists notice:

• 2-cell embryo
• 4-cell embryo

The cells divide evenly and multiply in an embryo’s normal growth. While this may appear easy under the microscope, what happens at the microscopic scale is incredibly complex.

Embryo Grading on Day 2

The evaluation of embryos is based upon:

• Cell equality
• Cell structure
• Fragmentation

Understanding Embryo Grades

What is Fragmentation?

Fragmentation is the term used to describe tiny fragments of cells that form within the embryo.

• Less fragmentation = healthier embryo
• More fragmentation = lower embryo quality

When a healthy embryo is developing, embryologists favor embryos that have equal and clear cells.

On Day 3, the embryo continues to divide rapidly. At this point embryos can include:

• 6 cells
• 8 cells

The embryo is graded in accordance with:

• Cell symmetry
• Percentage of fragmentation
• Pattern of growth

Examples of grading are:

• 6A
• 6B
• 8A
• 8B

A healthy embryo’s development on Day 3 typically has cells which are evenly divided with little fragmentation.

Signs of a Good Day 3 Embryo

• Equal-sized cells
• Proper cell division
• Minimal fragments
• A smooth appearance

Fragmentation Percentage Guide

The embryo’s development quality these days can help embryologists to determine the best embryos to be transferred for further growth.

Day 4 is a critically crucial stage in the development of embryos.

The amount of cells has grown substantially. Instead of different cell types, the cells begin to join and form a unified mass. This stage is referred to as:

• Compaction Stage
• Morula Stage

What Does a Morula Look Like?

Instead of a single cell, embryologists look at:

• A clustered mass of cells
• Compact structure
• Dense embryo formation

At this point the embryo begins to prepare its body for an important phase, known as the blastocyst stage.

Features of Day 4 Morula Stage

The development of a healthy embryo during the morula phase is very important for the successful IVF results.

Not all embryos develop in a similar way. Embryologists carefully check the embryo quality before deciding which embryos are suitable for transfer. 

Generally Preferred Embryos

A Grade Embryos

• Best quality
• Equal-sized cells
• No fragmentation
• Highest potential for implantation

B Grade Embryos

• Mild fragmentation
• Still in a position to be successful with a pregnancy
• It is commonly used in IVF

C and D Grade Embryos

• Higher fragmentation
• Lower quality
• Sometimes, it is monitored more closely up to Day 5 or 7

Sometimes even lower-grade embryos continue healthy embryo development and become blastocysts later.

• Identify healthy embryos
• Increase your chances of implantation
• Increase in IVF success rates
• Choose embryos that have better potential for growth

Origyn IVF has advanced IVF labs and skilled embryologists who closely monitor every phase of embryo development to provide individualized fertility care.

Factors That Affect Embryo Development

Several factors influence healthy embryo development, including:

• Egg quality
• Sperm quality
• Maternal age
• IVF lab conditions
• Genetic health
• Culture media quality

Even if embryos appear flawless externally, genetic aspects play a significant factor.

Following Day 5, the embryo usually attains the following:

• Blastocyst stage (Day 5 to 7)

A blastocyst has:

• An inner cell mass
• Cavity filled with fluid
• Outer cell layer

This is typically the most preferred method for embryo transfer in a lot of IVF instances. The development of embryos successfully up to the blastocyst phase which increases the chance of implantation and the possibility of pregnancy.

The initial four days of embryo development are crucial in the IVF process. From fertilization to the morula stage, every day offers embryologists valuable details about the quality of embryos and potential growth.

Knowing these phases will aid couples in getting connected to their IVF journey, and decrease anxiety throughout the treatment.

At Origyn IVF, fertility experts and embryologists use modern monitoring techniques to aid in healthy embryo development and boost IVF success rates.

For more information about IVF fertility and the growth of embryos, you can also follow Origyn IVF on Instagram and keep up-to-date with fertility advice from experts along with patient-education.

1. What is embryogenesis in IVF?

The term “embryo development” refers to the process by which fertilized eggs divide, and develop into an embryo in the IVF lab prior to the transfer of the embryo.

2. How many embryos should they be able to accommodate by Day 3?

An ideal Day 3 embryo usually contains up to 6-8 cells and very little fragmentation.

3. Do B-grade embryos have the potential to result in the birth of a baby?

Yes. B-grade embryos can produce healthy babies and are often used in IVF treatment.

4. What is the definition of embryo fragmentation?

Fragmentation is the term used to describe tiny fractured parts of cells visible within embryos. More fragmentation can affect the quality of an embryo.

5. What happens next after Day 5 in the development of embryos?

On Day 5, the embryos typically move towards the blastocyst stage from Day 5 through Day 7 before freezing or transfer.