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Understanding Embryo Grading & How Embryologists Grade Embryos

Embryologists and fertility doctors need an objective assessment guide to help them decide on the best embryos, the ones most likely to result in a pregnancy and a healthy baby, to transfer during the process of in vitro fertilization (IVF).

The number and quality of embryos a fertility doctor transfers influence the procedure's success rate. Transferring low-quality embryos may cause uterine implantation failure and ultimately hinder pregnancy.

This article discusses embryo grading, its criteria, the various grading systems available, and the role of embryo grading in IVF success rates.

What is Embryo Grading?

embryo grading

Embryo grading is a scoring system used by embryologists or fertility doctors to determine the quality of embryos before their use in treatment. Embryo grading provides the clinic with a priority guide as to which embryos to transfer.

During IVF, eggs are artificially inseminated inside the laboratory. After successful fertilization, the fertilized egg divides and grows to become an embryo. Before the embryos are utilized, the embryologist conducts a quality assessment using an embryo grading system. This grading system allows the embryologist to determine which embryos should be used for treatment and which should not be.

Is Embryo Grading Beneficial?

A study published in the US National Library of Medicine revealed that embryos with a higher grade had better clinical pregnancy and live birth rates compared to those with a poor grade.

Apart from an embryo grade, factors such as the genetic makeup of the embryo, laboratory conditions, experience, and skill of the fertility expert can influence the overall success of an IVF treatment. Genetics is the gold standard for embryo choice and truly drives success.

Notwithstanding, embryo grading still plays a role. Selecting only good-quality embryos provides the best opportunity that an embryo will be able to endure various types of possible treatments (e.g., PGT biopsy, cryopreservation, and thawing).

"Preimplantation genetic testing (PGT) is a more objective test while embryo grading comes from an embryologist’s subjective opinion," according to an Advanced Fertility Care embryologist. In recent times, PGT has taken over because we know that it is far more important than my subjective opinion," the embryologist further explained.

Criteria for Embryo Grading

A few hours after fertilization, the fertilized egg or zygote starts dividing to form a mass of cells that develops gradually into an embryo. The embryo cell number progressively increases within the first few days. 

The embryo cell number should be 2-4 cells after 48 hours and up to 7-10 cells by 72 hours.  Embryos are most commonly cultured to the blastocyst stage, which occurs between the 5th and 7th day of culture.

Grading Systems

There are different embryo grading systems, each focusing on specific characteristics of the dividing cells or blastocysts.

The IVF embryo grading system focuses on the following features:

  • degree of fragmentation and symmetry
  • the expansion rate and hatching of the blastocysts
  • inner cell mass
  • trophectoderm (a group of specialized tissues that differentiate and form the outer cell layer of a developing embryo.)

Generally, embryologists use the grading system based on the blastocyst expansion, inner cell mass, and trophectoderm for day five embryo grading. While day three embryo grading is typically based on the degree of fragmentation and symmetry.

Here are the common embryo grading systems most fertility clinics use to assess embryo quality before its transfer into the uterus for implantation.

Embryo Grading Based on Blastocyst Expansion and Hatching

Fertility specialists use the expansion rate of the dividing cells to score the embryo. It is also a numeric scoring system, and each embryo receives a score from 1-6.

The following is the numeric scoring system and its description using the blastocyst expansion and hatching feature:

  • 1: Blastocyst development and stage status
  • 2: Blastocoel cavity occupying more than half the volume of the embryo
  • 3: Full blastocyst with cavity completely filling the embryo
  • 4: Expanded blastocyst with cavity larger than the embryo and thinning of the zona pellucida
  • 5: Hatching out of the zona pellucida
  • 6: Hatched out of the zona pellucida

Embryo Grading Based on Inner Cell Mass

The inner cell mass is the group of cells in the developing embryo that transforms to produce the cellular structures that form a fetus.

Embryo grading using the inner cell mass is an alphabetical scoring system and includes the following:

  • A: Many cells, tightly packed
  • B: Several cells, loosely grouped
  • C: Very few cells

In this system, the quality of the inner cell mass reflects the quality of the embryo.

Embryo Grading Based on the Trophectoderm

A trophectoderm is a group of cells that are the first to differentiate and form the outer cell layer of a developing embryo.

Embryo grading using the trophectoderm entails:

  • A: Many cells forming a cohesive layer
  • B: Few cells forming a loose epithelium
  • C: Very few large cells

In this system, the embryologist uses the quality of the trophectoderm to assess the quality of the embryo.

Significance of Grades in Predicting IVF Success

Our embryologist also noted that: "just because an embryo looks good doesn't mean it's genetically normal, and just because it is a grade B doesn't mean it isn't normal. Today we use embryo grades as a way to demonstrate why an embryo was chosen to biopsy and/or freeze. The reason we do this is that poor quality embryos can't survive a biopsy and they also cannot survive the freeze and thaw process either."

Embryos with good grades have the potential to increase the chances of achieving pregnancy during an IVF treatment. In contrast, poor-quality embryos may fail to implant into the uterus, inevitably limiting IVF success.

Therefore, embryo grading provides fertility experts with the tool to make objective clinical decisions on the embryos to transfer.

Understanding Your Embryo Grade Results

Embryo grading will begin on day five. The embryos will be graded each day between day 5 and day 7. Any embryos deemed to be good quality will be utilized in treatment that day.  

Any embryos that are not chosen are allowed to continue to develop until day 7. The IVF lab routinely expects that approximately 50% of the embryos in culture will make good-quality embryos for use.

Interpreting Your Embryo Grading Report

Embryologists typically assign alphanumeric scores, such as 1AA, 2BA,3BC, etc., to embryos to indicate the quality level.

The following are possible embryo grade scores according to their exhibited degree of blastocyst expansion, inner cell mass, and trophectoderm grades:

  • High-quality embryos: ≥3AA
  • Poor quality embryos : 1–6BC, 1–6CB, 1–6CC

Of note, these embryo grades alone do not indicate failed IVF treatment. Other factors, such as an embryo's genetic makeup, medical history, or age, among other things, also affect your chances of achieving IVF success and pregnancy.

Do You Need Help with IVF Treatment?

Embryo grading is one of many tools that provides crucial information to fertility doctors to enable them to select suitable embryos for transfer. Transferring poor-quality embryos can reduce the success rate of IVF treatment.

If you intend to go for IVF treatment, consult an experienced fertility doctor and learn more about their embryo grading process.

Our team at Advanced Fertility Care has proven experience with IVF, and we use standardized embryo grading methods to assess quality before embryo transfer.

If you want to learn more about our process and IVF treatment, contact us or schedule an appointment with our team today. We look forward to supporting you in starting or building your family using IVF. We serve in different areas.

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