Stages of embryo development

A good, healthy embryo is integral to the whole process of conception and healthy pregnancy and naturally, when you do IVF, there’s a lot of focus on the embryo. Informed patients today are aware of the importance of embryo and are asking questions about embryo grading in IVF.

So, which embryos are most likely to result in a healthy pregnancy?

During the IVF process, embryos created in the lab are studied under the microscope and graded by the embryologist on the basis of their appearance. This grading allows the doctors and embryologists to differentiate the good embryos from those with average or poor quality, so they can choose the ones with the highest potential to result in a healthy baby.

This grading is simply based on the appearance of the embryo under a microscope. While it gives a fair idea of which embryos are more likely to survive, it does not give conclusive evidence to the embryo being of the “baby-making” variety as we don’t know about the genetic contents.

However, unless you are opting for advanced pre-implantation genetic testing of embryos, understanding the embryo grading chart should be helpful in learning about the quality of your embryos and possibility of success in IVF.

Day 3 embryo grading

Embryos at day 3 are also known as ‘cleavage stage’ embryos as the cells within them are dividing (cleaving) as the genetic material within them replicates, but the embryo at this stage is not growing.

Stages of embryo development

It is important to note here that embryos do not divide in a systemic, synchronous manner with one cell dividing into two, two into four, four into eight and so on. It is common to see embryos with five, six, seven cells, which does not indicate any abnormality.

Sometimes, while diving, a small portion of the cell’s cytoplasm separates and remains as an individual fragment inside the embryo. These fragments of cytoplasm do not contain nuclei but only the lost cytoplasm. While not much is understood about the causes of fragmentation, the embryos with too many fragments are considered poor because their cells have lost too much mass to be healthy.

Embryo grading at day 3 is done on the following three criteria:

  1. The number of cells in the embryo: A good-quality, normally-growing, embryo on day 3 would typically have 6-10 cells held within an outer shell called the Zona Pellucida (ZP). Research shows that embryos with 8 cells or more are most likely to develop into a healthy blastocyst. Different clinics grade embryos upto 4 or 5 points, with 5 being the lowest.
  2. Fragmentation: Embryos with no or little fragmentation are labeled grade A, those with more than 10% fragmentation are grade B, and the ones with more than 25% and 50% fragmentation are labeled C and D grade respectively. Embryos with grade A and B are considered good and most likely to result in a healthy pregnancy.
  3. The appearance of cells under a microscope: Embryos with cells of equal sizes and no fragmentation are considered grade 1 or the best quality. Some laboratories label embryos as:

G (good) — all cells are equal size

F (fair) — a few of the cells are different sizes

P (poor) — cells are mostly all different sizes

An embryo with label 8AG will be considered the best. However, it must be noted that the symmetrical size of cells is not very significant in determining the quality the embryo or their growth potential.

Ideally, on Day 2 the embryo should have 4 cells and by Day 3, it should have 7-9 cells. According to some researchers, this cell number is perhaps the most important factor for determining the implantation potential of cleavage stage embryos.

Grade 1 embryo

It was also found (4) that faster growing embryos (those with higher number of cells on Day 3) has relatively lower implantation rate. Embryos having more than 9 cells on Day 3 were likely to have chromosomal abnormalities (5).

Day 5 embryo grading

Blastocyst grading is mainly done using the Gardner system (Gardner and Schoolcraft 1999).

By day 5 as the embryo continues to grow, the number of cells increases and at this stage the cells also start to differentiate into specific cell types. By this time, the outer covering (ZP) becomes thinner, so the blastocyst can burst through it an implant into the uterine lining.

The day 5 embryo (or blastocyst) has 2 kinds of cells:

  1. Inner cell mass (ICM), which will eventually form the fetus
  2. Trophectoderm Epithelium (TE), which will form tissues needed during pregnancy (like the placenta)

A blastocyst is like a fluid-filled sac with the TE cells on the outside and the ICM on the inside.

Both of these cell types are essential for a healthy pregnancy as they form the fetus and the placenta.

So, blastocyst grading involves assigning a separate letter grade to each cell type as well as to the cell cavity or blastocoel, which is filled with fluid. The size of the embryo and the number of cells in it are also noted at this point.

The three parameters of Day 5 embryo grading

Number: Based on the expansion of blastocyst

The expansion of a blastocyst gives an indication of its ability to develop and grow. An embryo that does not expand, will not be able to grow.

1  Early blastocystBlastocoel or the cavity comprises less than 50% of the volume of the embryo
2BlastocystCavity takes more than half the volume of the embryo
3ExpandingThe outer membrane has thinned and the cavity is fully expanded
4HatchingThe embryo has expanded and is starting to burst through the ZP
5Completely hatchedThe embryo has burst out of the ZP

First letter: Grade of the inner cell mass (ICM), which will form the fetus

AWell-defined, cohesive layer of many cells
BCells are loosely packed; may have a grainy appearance
CFew large, dark cells that seem degenerative

Second letter: Grade of the trophectoderm (TE), which forms the placenta and other supporting cells

AMany equal-sized, tightly packed cells that give a neat appearance
BIrregular, loose layer of cells
CFew cells placed very irregularly; maybe dark or grainy

In a very early blastocyst, the cavity is just starting to form and the cells cannot yet be differentiated into the two types.

An expanded blastocyst has a well-defined cavity, around 100-125 cells all covered with a thin ZP.

The Hatched Blastocyst has over 150 cells with the embryo bursting out of the shell.

Embryo grading and IVF success rates

Embryo grading is not an absolutely accurate science, but it does give an indication of the possibility of its survival.

However, there’s nothing like best embryo grading for transfer as we cannot determine the exact usability of an embryo simply by looking at a grading chart.

For instance, an embryo with 1CA grade on seem poor at first, but it is possible that it may still be developing and may turn into grade 4AA within a day.

So, you have frozen a few embryos of quality say 4AA, 4CB and 5BC and the best quality one doesn’t result in a live birth, it doesn’t mean all hope is lost. The other embryos may still result in a positive pregnancy and a healthy baby.

While it is helpful to know about your embryos’ grading, you should not be too disappointed if there are few seemingly-good embryos.

Infact, researchers are now looking into other factors such as metabolomics that could impact the quality of an embryo.

Success with C grade embryos

It might be surprising that even though C grade embryos are usually discarded or not preferred for transfer, there is growing evidence that these embryos may actually result in positive outcomes:

Research published in the Journal of Reproductive Biomedicine Online, found that live birth rates with embryos given C grade were 34.6% as compared to 39% for grade B and 46.8% for grade A. The average age of women in this study was 30 to 32 and it was founded that the seemingly poorer embryos did not cause any increase in miscarriage.

So, you cannot decide the absolute fate of an embryo simply based on the way it looks.

Implantation failure

According to some researchers, the embryo quality accounts only for one- third of implantation failures, while the rest are caused by poor endometrial receptivity and inefficient embryo–endometrial interaction.

Embryo grading is one step in your IVF journey and it is certainly not the most important one. There are many other things to consider such as the decision to transfer a fresh vs frozen embryo, the development of your lining, how many embryos to transfer, whether or not to do an ERA test, and post-transfer management.

Your doctor and medical team will consider all these factors along with your health and fertility history before implanting an embryo into your uterus.

For affordable IVF and fertility consultations with some of the top doctors in the world, get in touch via the form on this page.


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