One of the most exciting stages of the in vitro fertilization (IVF) treatment cycle is learning that you have viable embryos. You may end up with multiple embryos to work with after your egg retrieval and fertilization. However, you won’t be implanting all of them. Before your embryo transfer can take place, you’ll need to narrow down which embryos have the best chance of implanting. Choosing which ones to transfer, which ones to freeze or donate, and which ones to discard is a decision you’ll make with your infertility specialist. To help you make the best decision, you’ll need to understand the role embryo quality plays in IVF success rates.
(For more in-depth information about embryo grading, check out our guide What is embryo grading?)
Conception, or embryo implantation, is the moment when an embryo successfully attaches to the endometrium. This usually occurs between days-20 and -24 of a 28-day menstrual cycle. In an IVF treatment cycle with a fresh embryo transfer, it happens six to ten days after your egg retrieval, and one to five days after your embryo transfer procedure. If you are undergoing a frozen embryo transfer (FET), conception happens one to five days after your embryo transfer procedure.
There are two major factors that contribute to successful embryo implantation. First, your endometrium must be receptive, with adequate thickness to support implantation. Second, embryo quality has to be sufficient for the endometrium to invest resources and nutrients in developing it further. If the endometrium does not deem the embryo to be healthy enough, it will prevent implantation.
It’s important to remember that even though IVF and embryo transfers can overcome a lot of barriers to getting pregnant, the embryo has to attach itself to the endometrium on its own. Even if you’ve done everything right, there is still a chance for the implantation to fail.
Embryologists determine embryo quality by using the Gardner Embryo/Blastocyst Grading System. This system provides the rubric for how embryos are graded. As your fertilized eggs mature in the lab, there are two stages when they may be evaluated: on the third day after egg retrieval and again on the fifth day.
Using a high-powered microscope, embryologists assess and evaluate how your embryos are developing. They will assign a grade to the various aspects of development. Your infertility specialist will walk you through your embryo grades, what they mean, and give you their opinion on how to proceed.
It should be noted that individual embryologists may interpret some of the visual variables of embryo grading differently. Your embryo may also continue to develop into a higher quality after it has been assessed, so embryo grades are never set in stone.
The first time your embryos are graded is on the third day after your egg retrieval. At this stage, the embryos will have undergone compaction, and the cells are starting to divide. This is usually the earliest your embryos can be considered viable for implantation.
To grade your embryos, embryologists evaluate their overall structure, or morphology, and count how many cells there are. More specifically, embryologists judge whether the cells are all the same size, whether they contain nucleuses, and how much fragmentation has occurred as a result of cell division. They also count how many cells are present. Eight to ten cells is ideal when implanting day-three embryos, but it’s important to remember that cells will continue to divide after embryos have been graded.
Assuming you didn’t go ahead with a day-three transfer, your embryos will be graded for the second time on the fifth day after your egg retrieval. Grading day-five embryos is more complex. As the embryos are entering their final stages of development, there are more factors to be evaluated. When looking at your embryo grades, it’s important to remember that because of this complexity, the grades may change as the embryo continues to develop.
The following are the three main aspects embryologists evaluate:
When you receive your embryo grades, they will each have a number to indicate blastocyst expansion, along with two letters each indicating ICM and TE quality. Your infertility specialist will recommend which embryos they believe offer the best chances of pregnancy and a live birth.
Once grading has been completed, your infertility specialist and embryologist will decide which embryos they think are the best candidates for the embryo implantation process. Part of this decision-making process involves weighing the medical history of both parents, as well as morphology and the rate of development.
The age and medical history of both parents are important, especially with older mothers as egg quality declines with age and affects embryo quality. Fertility specialists generally won’t recommend implanting embryos that don’t meet “excellent” or “good” quality standards in these instances.
Embryo morphology refers to its overall structure. Embryo grading is largely based on morphology, as the way an embryo looks can be a strong indicator to help gauge its potential for successful implantation and gestational development. The following chart breaks down how infertility specialists commonly categorize the different blastocyst grades:
Grading Pregnancy Rate Live Birth Rate Excellent 3AA 4AA 5AA 65.0% 50.0% Good 3AB 4AB 5AB 3BA 4BA 5BA 59.3% 49.7% Average 3BB 4BB 5BB 50.3% 42.3% Poor 4BC 5BC 4CB 5CB 33.3% 25.0%
An equally important factor is how quickly an embryo moves through the stages of development. Most experts agree that the speed of development correlates with odds of success. This is why it’s possible for your infertility specialist to still recommend implanting an embryo with an “average” grade, if it develops with a relatively quick pace compared to others.
If you are planning to freeze your embryos, you should know that typically only high-quality embryos will be chosen. Due to the nature of how they are stored, there is a minimal risk of degradation. This is why low-grade embryos are generally not considered for freezing.
When embryos are frozen, their development is halted for the length of time they’re in storage. Once they’ve been thawed, they resume developing. So, it’s possible for your embryo’s grade to change prior to implantation.
A lot of emphasis is put on embryo grading, but it’s only one variable in successful implantation. Maternal age and synchrony between embryo and endometrium matter just as much, if not more. If you find yourself looking at a less-than-stellar roster of embryos, don’t give up hope.
Embryo grading is not set in stone. Every embryologist is going to interpret visual results differently, and your embryos will continue developing once they’ve been graded. “Low-quality” embryos may have other factors going for them, like a fast rate of development or a low presence of fragmentation resulting from cell division.
Most importantly, low-quality embryos still have a chance to implant successfully and result in a live birth. You can rest assured that your fertility specialist will walk you through all of this when the time comes.
Fundamentally, embryo grading is only evaluating the physical appearance of your embryos. While physical appearance can tell us a lot about an embryo, it can’t tell us anything about the overall genetic health of any given embryo. The only way to assess the genetic health of your embryos is to send them for preimplantation genetic testing (PGT).
The genetic health of an embryo becomes a deciding factor in conception and live birth when aneuploidy is present. This is a condition where the pairs of chromosomes that make up human DNA don’t add up to the standard 46. It can lead to miscarriage, or in the case of live birth, conditions like Down Syndrome and Turner Syndrome. As you get older, the chances of aneuploidy increase. Thankfully, the availability of preimplantation genetic testing for aneuploidy (PGT-A) makes it possible to test for aneuploidy before you have your embryo transfer.
