Embryo Grading and Success Rates
Once fertilized, embryos are graded according to their form and quality. The most common scale uses a number and two letters to describe each embryo (‘4AB’ for example). Transfers using excellent grade blastocysts have a pregnancy rates of about 65%. When the blastocyst grade is average, studies show the pregnancy rate drops to 50%. Poor quality blastocysts may achieve a pregnancy rate of just 33%. You can find your own embryo grade in the chart on the page below.
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Embryo Development from Zygote to Blastocyst
After your eggs are retrieved and fertilized, you will get an IVF report with embryo grading. The report explains how many eggs were fertilized, how many embryos were conceived, and how many made it to the coveted “Blastocyst stage”. The IVF reports are different from clinic to clinic, and they can be very hard to interpret and (more importantly) know if you have quality embryos and a good shot at a healthy pregnancy. Your IVF report may reference these major stages of embryo development at various days following the fertilization:
• Zygote: When a sperm and egg meet, fertilization occurs. At this point, a zygote is formed.
• Compaction: Just prior to forming a morula is a “compaction” stage. At this time an 8-cell embryo undergoes a series of changes that transform the zygote into a solid ball of cells.
• Morula: A grouping of a solid mass of 12 -15 cells resembling a blackberry. This forms around four days post-fertilization. The morula phase is the final phase prior to blastocyst development.
• Blastocyst: A blastocyst is the phase just before the embryo latches onto the uterine wall and “hatches” into a fetus. Blastocysts develop five days after the initial fertilization and contains 3 distinct features. These features include a round shell, a fluid-filled cavity and an internal cell mass that will become the fetus.
• Hatching Blastocyst: Around six days after fertilization, a healthy blastocyst will hatch from its protective shell to implant in the uterine wall. If the blastocyst develops beyond this stage in the lab, it’s no longer viable. If the embryo successfully attaches to the uterus, then you have a pregnancy.
The normal timeline for embryo development is after two days of cultivation the embryo should have between 2-4 cells, then 6-8 cells by 3 days. By Day 4 the cells should begin to clump together to form the Morula. The blastocysts should develop by Day 5 or 6. By day 7 the blastocyst should start hatching. Once hatched, the embryos is not viable for transfer.
What is a Blastocyst?
Knowing the components of a blastocyst is key to understanding its grade and likelihood of a successful pregnancy.
A blastocyst is distinguished by its two clearly defined cell components and a fluid cavity. The quality of the blastocyst describes how clearly formed are these structures. As development of a Blastocyst progresses, these two cellular masses divide and the fluid cavity enlarges. The Inner Cell Mass (ICM) will become the fetus, the Trophectoderm (TE) will become the placenta. Both should be well formed for the pregnancy to be successful.
Blastocyst transfer can have high pregnancy success rates with very low risk of multiple births. Many IVF doctors will do embryo transfer only once the embryo reaches Blastocyst stage, although some others prefer to transfer sooner and let the embryo reach Blastocyst stage within the uterus.
The Gardner Embryo/Blastocyst Grading System
The most common system for ranking the quality of blastocyst embryos is the Gardner grading system. According to the Gardner system, each blastocyst embryo is assigned 3 separate quality scores:
1. Blastocyst development stage – a range from 1 – 6 (with 5 being the most developed)
2. Inner cell mass (ICM) score, or quality – range A – C (A being the best)
3. Trophectoderm (TE) score, or quality – range A – C (A being the best)
So the least advanced blastocyst would be 1CC, and the most would be 6AA. However it’s generally preferred to do embryo transfer as they reach level 5 (which means it’s just starting to hatch). So we look for Blastocysts that are level 4 or 5 with letter grades of A or B.
To understand the quality of your embryos, you can refer to this simple chart:
|Blastocyst Stage||Quality Grade||Description|
|Early blastocyst||1||The blastocyst cavity is less than half the volume of the embryo|
|Blastocyst||2||The blastocyst cavity is greater than or equal to half of the volume of the embryo|
|Full blastocyst||3||The blastocyst cavity completely fills the embryo|
|Expanded blastocyst||4||The blastocyst cavity volume is larger than that of the early embryo and the surrounding membrane is thinning|
|Hatching blastocyst||5||The outer layer of cells has started to herniate through the surrounding membrane|
|Hatched blastocyst||6||The blastocyst has completely escaped from the surrounding membrane|
|Inner Cell Mass||A||Tightly packed, many cells|
|Inner cell mass||B||Loosely grouped, several cells|
|Inner cell mass||C||Very few cells|
|Trophectoderm||A||Many cells forming a tightly knit epithelium|
|Trophectoderm||C||Very few cells forming a loose epithelium.|
Not all embryos reach the Blastocyst stage within 5 or 6 days. Some are slow to develop and take longer to become Blastocysts (while others simply stagnate and don’t develop further). Slow developing embryos may be preserved by the clinic depending on their apparent health.
Embryo Grading and Success Rates
So what does all this mean for your surrogacy journey? Blastocysts that are well developed and with healthy internal structures are more likely to develop into successful pregnancies. According to various studies, transfers using excellent grade blastocysts (greater than 3AA for example) the clinical pregnancy rates was about 65%. When the blastocyst grade was average (for example 4BB, 4AC, 4CA, 2AB, 2BA) the pregnancy rate was about 50%. Poor quality blastocysts (3BC, 4CB, 4CC, 2BB) achieved a pregnancy rate of just 33%.
Consider the chart to the right. The most desirable blastocyst grades are those in blue. These have the greatest likelihood of pregnancy. Those in the orange area would be considered poor. Blastocysts in the red areas would not be viable and likely discarded. Of course the actual likelihood of pregnancy depends on more than just the quality of your embryos — the correct preparation of your surrogate and the skill of the embryologist has a lot to do with the ultimate success. But this is a good guide to the relative value of the embryos presented in your Day 5 IVF report.
Embryo Early Stage Transfers
For those embryos that do not make it to Blastocyst stage by Day 5 or 6, if they reach the stage immediately before becoming Blasts they may be considered Early Blastocyst stage. These are less developed than Blastoscysts, but still have the possibility to develop into blasts within the uterus (although the chances are less likely). And the Blastocyst grading scale does not yet apply because the embryo doesn’t have the structures in place that define the grade of a Blastocyst. For these, the grade is based on the number of cells and the a few other criteria (fragmentation and nucleation for example). Clinicians give a grade from 0 to 3 (3 being the best formed).
The stage immediately before Early Blastocyst is called the Compact Stage (or even the Early Compact stage). Embryos that don’t make it past this point after 5 days are less likely to develop further and many clinics will discard these. Even though they may be eventually discarded, clinicians may still give a grade similar to Early Blasts, with the number of cells and a grade from 0 to 3.
Day 3 Embryos
Most ART cycles will transfer embryos on Day 5 when they reach Blastocyst or Hatching Blastocyst stage. But some clinics still prefer to transfer embryos during the earlier stages, often at Day 3. The strategy is that the embryos develop more normally in the uterus than in the laboratory, so there is higher likelihood that embryos will reach a healthy blastocyst stage if transferred into the uterus early.
Embryos are graded differently at this stage. Day 3 embryos are rated on an A, B, C and D scale, which reflects the rate of development on that particular day, the fragmentation percentage, synchrony of cell division, and evenness of cell division.
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