Grading of IVF Embryos

Standardized Embryo Grading Systems

There are many embryo grading systems that embryologists use to evaluate embryos for transfer. The ASRM (American Society of Reproductive Medicine) has attempted to establish a standard grading system, so when a patient or previous clinic is talking about "the quality of the embryos", it can be interpreted and understood by the clinic she is now at, which is important in trying to understand why her cycle of IVF was unsuccessful and what "changes" can be made to result in a successful outcome. Even with standardized grading systems, which is still not universally used, evaluation and scoring of any stage of embryo development, from fertilization to a hatching blastocyst is critical to the experience and expertise of the embryologist.

The Embryo Grading Process

After oocyte (egg recovery), the eggs are placed in the incubator for a number of hours before they are checked for maturity. At that time, the embryologist will remove the cells surrounding each egg (cumulus cells) and check the egg for maturity. Each mature egg will undergo ICSI, and be placed back in the incubator. The following day the embryo will be checked for fertilization but is not graded. The embryos will not be checked again until day 3, 4, or 5 (blastocyst) depending on when the transfer was planned. At that point in time, the embryologist will check each embryo and "grade" it, picking the best for transfer, and he will determine if the remaining ones are suitable for cryopreservation.

Day 3 Embryo Grading

Grading of a day 3 embryo is based on the number of cells that make up the embryo, the amount of fragmentation, and the symmetry of each of the embryo's cells (blastomeres).

• An embryo should be between 7-9 cells on day 3, with an ideal embryo being 8 cells. However, a 7-cell or 9-cell embryo can be perfectly fine and result in a successful pregnancy. In Arizona Center for Fertility Studies experience, embryos that are 5-cells or less, have a significantly lower chance of achieving a successful pregnancy. A 6-cell embryo may be okay, and should be considered for transfer if there are not embryos of higher cell number. At Arizona Center for Fertility Studies, we have had successful pregnancies from the transfer of 6-cell embryos, however, they are statistically lower than if the cell number was higher.
• Fragmentation is when parts of an individual cell(s) in the embryo break apart or "fragment" and appear as small fragments or "blebs" within the embryo. In Arizona Center for Fertility Studies experience, the degree of fragmentation is directly related to the overall quality of the embryo. If an embryo has no fragmentation it is scored an A, if the embryo has less than or equal to 10% fragmentation it is scored a B. Embryos with a greater percentage of fragmentation are scored C or D. Generally, when embryo fragmentation is scored A or B, it is considered to be a very good embryo and suitable for transfer. C scored embryos can be transferred but statistically result in lower pregnancy rates. At Arizona Center for Fertility Studies, if an embryo is scored with D fragmentation, it is not recommended for transfer or cryopreservation. It is put back in culture (as Arizona Center for Fertility Studies always does), and experience shows that these embryos begin to degenerate over the next day or so.
• Symmetry is assessing the cells in the embryo is see if they are all the same size, or symmetrical. If all the cells in the embryo are the same size, the embryo is rated G (good). If a few of the embryo's cells are different sizes it is scored F (fair); and if most of the blastomeres are of all different sizes, it is scored a P (poor). Embryos that are scored G or F are suitable for transfer, and along with the other scoring criteria, help to predict success. Embryos scored D, are not recommended for transfer or cryo, and almost always are associated with decreased cell numbers and increased fragmentation. Again, they are placed back in the incubator, and within 1-2 days undergo significant degeneration. In Arizona Center for Fertility Studies experience, symmetry, or lack of perfect symmetry, is not as significant as cell numbers and fragmentation. Many times, embryos with less than perfect symmetry, have resulted in successful pregnancies.
• Other factors that go into evaluating an embryo for transfer but are not part of the scoring system is appearance of the zona pellucida (does it appear "hardened" (see assisted hatching link), whether or not the cytoplasm of the embryo is homogenous or granular (or grainy) and the presence of vacuoles inside the cytoplasm of individual cells. Vacuoles are the walled off areas of waste material produced by the embryo. A large number of vacuoles within an embryo, statistically, is indicative of a lesser quality embryo but, as an isolated factor, may not necessarily indicate that the embryo would not result in a successful pregnancy. It is only when increased vacuolization and/or increased cytoplasm granularity, along with decreased cell numbers, fragmentation and asymmetry, does it predict decreased success rates with embryo transfer.

Different stages of scoring a day 3 embryo-evaluating the number of cells, amount of fragmentation and symmetry of the cells (blastomeres)
(Bączkowski et al., Reproductive Biology, volume 4 No. 1, pgs. 5-22)

In summary, a "perfect" day 3 embryo should be 8AG - eight cells, no fragmentation and all the cells are equal and symmetrical. Human embryos are so variable, and although scoring is very important in helping to decide which embryos to recommend for embryo transfer and which to freeze, embryo quality as evaluated under the microscope, even by very experienced embryologists, ultimately is only a reasonable "estimate" of predicting the chances for a successful pregnancy. Any good embryologist will tell you that "even though an embryo(s) looks good, it doesn't mean that it is good" and visa versa.

Blastocyst Grading

Scoring of blastocysts are similar but have a completely different set of scoring criteria. Blastocyst quality is determined by evaluating the outer ring of cells, known as the trophectoderm or trophoblastic cells, that will eventually form the placenta; the inner cell mass or ICM, which is made up of the stem cells, that the baby will develop from; and finally, the degree of expansion of the blastocyst cavity and whether or not it has started to "hatch" or break out of its zona pellucida or "shell". Again, scoring of a blastocyst depends on the experience and expertise of the embryologist. Like day 3 embryos, scoring of even blastocysts is an imperfect science, and some very nice looking blastocysts do not produce a pregnancy; whereas, less than optimum ones do. However, the basic rule of thumb is "that the best of the best embryos" make it to the blastocyst stage, and a very nice blastocyst has a greater chance statistically of producing an ongoing pregnancy than a lesser quality one.

The different grading of a blastocyst-evaluating the outer ring of cells or trophectoderm, the inner cell mass (ICM) or stem cells and the expansion of the blastocyst cavity or blastocele. A "perfect" day 5 blastocyst would be scored 4AA (expanded blastocele cavity, nice trophectoderm and numerous tightly packed cells in the ICM). Blastocyst score 5AA - is a blastocyst that is starting to "hatch" with normal appearing trophectoderm and ICM. A blastocyst with a score of 6AA is a completely "hatched" blast with normal appearing ICM.
(Bączkowski et al., Reproductive Biology, volume 4 No. 1, pgs 5-22)

A day 6 "hatching" blastocyst scored 5 AA, that is starting to come out of its zona pellucida or "shell".

Since there are many other contributing factors involved that we cannot see or identify, embryo scores are, at best, generalizations about "quality" and are often inaccurate. Many times, Arizona Center for Fertility Studies has seen cycles fail after transferring 2-3 "perfect" looking embryos; and many times see successful ongoing pregnancies after transferring only one "decreased quality" embryo. The actual "potential" of the embryo to implant and continue normal development is impossible to accurately measure and predict with current technology. Although some clinics will cancel the embryo transfer if there is only one embryo or several embryos of "poor quality"; for the above reason and based on extensive experience, Arizona Center for Fertility Studies will always recommend transferring one embryo, even if it did not get the "best" scoring. "Bad embryos" do not cause "bad babies", they cause no babies. Therefore, as long as there is even the slightest chance of success, Arizona Center for Fertility Studies recommends the transfer of "less than optimum" embryos.

One additional thing that is very important is the embryo transfer. A traumatic and/or bloody difficult transfer can result in decreased pregnancy rates probably by disrupting the endometrial lining, introducing blood into the uterine cavity, triggering an inflammatory reaction, or by just having the embryos exposed to the outside environment, as opposed to the incubator environment, for too long. At Arizona Center for Fertility Studies, several months before an embryo transfer, a mock transfer is done to measure the depth of the uterine cavity, the degree of angulation and direction of the cervical canal and the ease or difficulty of eventual embryo transfer. At the time of the actual embryo transfer there should be no surprises. At Arizona Center for Fertility Studies, embryo transfers take less than 30 seconds from the time the embryos are removed from the culture environment of the incubator to being loaded in the transfer catheter.

Ultimately, the true test of embryo quality is whether it results in a successful pregnancy and live birth. IVF is not an exact science, and when nothing has been overlooked, the woman has a reasonable FSH level and she is at a clinic with proven high success rates, than a successful ongoing pregnancy, is for the most part, with a few exceptions of very poor sperm quality even with ICSI, determined by the overall genetic quality of the egg. A thousand things can effect that, most of which we can not determine and/or correct. With controlled ovarian stimulation, Arizona Center for Fertility Studies tries to get as many eggs as genetically predetermined in each individual woman. The more eggs you get (balancing avoiding OHSS) the more possible embryos a woman has, the greater the percentage that the embryologist can find "good quality" embryos to transfer and enough to freeze. However, there are plenty of women that are "low responders"; they do not make a lot of eggs/embryos, but the ones they make are "excellent" and result in a successful pregnancy. But successful IVF may take more than one attempt. Each stimulation cycle is an independent cycle and is usually different than the previous one. If the first IVF cycle is not successful, Arizona Center for Fertility Studies has learned a tremendous amount of information about the woman and can change things with the next attempt. Thousands of previous cycles have "told" us what to do on that initial attempt, but every woman is unique and you can only conclude "generalizations" about what is the best protocol for any particular woman or age group.