Embryo Grading Day-by-Day

Whether you are looking for day 1 embryo grading information or on day 7 of embryo grading trying to compare your embryos, learning about the development of embryos can help you anticipate which of your embryos may be ideal for transfer.

This article provides a day-by-day description of embryo development, detailing how embryos are graded at each stage and the significance of these grades for embryo selection.

Fertility clinics can help you create embryos and determine embryo grades. Contact us to find a fertility clinic near you.

Day 1 Embryo Grading: Fertilization and Initial Cell Division

On day 1, fertilization occurs, and the sperm merges with the egg to form a zygote. The zygote begins its first cell division within 24 hours, typically resulting in two cells. At this stage, there isn’t much to grade since the embryo is still in its initial formation. However, some zygotes may stop developing even on day 1.

Overview Day 1

Ideal Development Stage: Fertilization and initial cell division.

Grading: Not applicable yet, as grading typically begins on day 2.

Day 2 Embryo Grading: Early Cleavage Stage

By day 2, the embryo has usually divided into four cells. This stage is referred to as the cleavage stage. In most cases, your fertility specialist will wait to grade embryos until day 3.

Overview Day 2

Ideal Development Stage: The embryo continues to divide, reaching 4-8 cells.

Grading: Minimal grading due to the embryo’s early development.  Embryologists may begin to observe cell symmetry and fragmentation.

Day 3 Embryo Grading: Cleavage Stage

On day 3, the embryo ideally reaches 6-8 cells. This is a significant milestone, and this is one of the most common times for an embryo to be graded.

Ideal Development on Day 3

Cell Number: 6-8 cells are optimal for grading.

Fragmentation: Observing the degree of fragmentation is important; lower fragmentation rates are indicative of better quality.

Symmetry: Embryos that are more symmetrical tend to be graded as higher quality.

On day 3, embryos are graded using a scale of 1-4 or A-D, with 1 or A being the most likely to develop into a blastocyst.

• Grade A or 1: No fragmentation, symmetrical.
• Grade B or 2: Minimal fragmentation, mostly symmetrical.
• Grade C or 3: Moderate fragmentation, asymmetrical.
• Grade D or 4: Heavy fragmentation, asymmetrical.

Day 4 Embryo Grading: Morula Stage

By day 4, the embryo has developed into a morula, a solid ball of cells typically consisting of 16-32 cells. Since Day 4 is between major developmental stages, clinics tend to grade on day 3 and day 5 more than on day 4.

Overview Day 4

Ideal Development Stage: The morula stage marks the transition to more complex structures.

Grading: Grading becomes more complex but fertility specialists commonly wait until day 5 to apply the more complex grading system.

Day 5 Embryo Grading: Blastocyst Formation

On day 5, the embryo develops into a blastocyst, a stage where grading becomes more detailed and significant. While embryos can be graded before this, it is harder predict which embryos will successfully develop between day 3 and day 5, so grading embryos at this time can give you a more accurate picture of what embryos may be successful as pregnancies.

Ideal Development Day 5

When fertility specialists grade day 5 embryos, they are looking for markers of blastocyst development that indicate the embryo is steadily growing and has distinct parts that appear healthy.

• Expansion/Hatching: The blastocyst expands and hatches from the zona pellucida, preparing for implantation.
• Inner cell mass (ICM quality): The ICM is the cluster of cells that will become the fetus.
• Trophectoderm (TE) quality: The TE consists of the outer layer of cells that will form the placenta.

Day 5 embryos are graded using a number and two letters. For example, an embryo may receive a 3AA grade, or a 1CC grade.

Higher numbers tend to indicate higher quality, and A and B letters indicate higher quality than C or D.

Expansion (1-6)

 This number represents how much the embryo has expanded and developed a cavity, called the blastocoel. Higher numbers generally indicate a more developed blastocyst.

• 1: Not expanded
• 2: Slightly expanded
• 3: Moderately expanded
• 4: Expanded
• 5: Fully expanded
• 6: Hatching

Inner Cell Mass Quality (A-C)

The ICM should consist of many cells clustered together. More cells indicate that the ICM is robust and optimal for developing further.

• A (High quality): Well-defined and compact
• B (Good quality : Fairly defined
• C (Low quality): Poorly defined

Trophectoderm Quality (A-C)

Cells in an ideal TE are arranged in a cohesive layer around the blastocyst. This uniform layer is essential for forming a healthy placenta.

• A (High quality): Well-developed
• B (Good quality): Moderately developed
• C (Low quality): Poorly developed

Significance of Day 5Grading for Embryo Selection

Grading on Day 5/6 is critical for embryo selection. High-quality blastocysts (like 5AA) have significantly higher implantation success rates. This makes them preferable for transfer, as they are more likely to result in successful pregnancies.

However, embryo grading is only one of several factors that affect success, including the age and health of the carrier.

If you have low-graded embryos, surrogacy may increase your chances of successful pregnancy. Contact a surrogacy professional now.

 If you have embryos ready, you can be matched with a surrogate in an average of 1 - 6 months. Learn more about available surrogates.

Day 6 and Day 7 Embryo Grading: Further Development

On Days 6 and 7, the embryo may continue to develop. However, the grading system remains similar to Day 5. The focus remains on the quality of the ICM and TE, along with blastocyst expansion.

Learn more about IVF success rates here.

View success stories here.

Frequently Asked Questions

Is 4AA or 5AA better?

A 5AA embryo is generally considered better than a 4AA due to its more advanced developmental stage and overall quality.

How long does it take to grade an embryo?

Embryologists may assess development daily, but grading may be shared with you less frequently, often during major milestones of development like Day 3 or Day 5.

How many good quality Day 3 embryos make it to Day 5?

The number of embryos that will make it to the blastocyst stage by Day 5 after being graded as high quality on day 3 varies widely. In some cases, more than half of embryos may stop developing between Day 3 and Day 5.

What percentage of embryos make it to Day 5?

In good cases, 50-60% of embryos may develop through day 5, but this can vary based on individual circumstances and clinic practices.