Which embryo should you transfer to give you the best chance of success in your IVF cycle? Choosing the right embryo to transfer can feel like one of the most important decisions of the process. If it doesn’t work, that’s yet another month you have to go waiting for another chance to get pregnant. Oftentimes, your clinic will recommend which embryo to transfer. But how do they decide? Thankfully, embryologists like Alease Barnes are here to guide you through it. In this post, we’ll break down how embryologists select the best embryo for transfer and what factors come into play.
Prioritize Day 5 Embryos over Day 6 Embryos
In most IVF labs, the top choice for transfer is typically a high-quality Day 5 embryo. Day 5 embryos, known as blastocysts, have developed to a point where they’re ready for implantation, giving them a strong chance of leading to a successful pregnancy.
An embryo that develops to the blastocyst stage on Day 5 is considered to have normal development. While Day 6 embryos still have excellent success rates, studies have shown that Day 5 tend to have an increased chance of IVF success compared to day 6 embryos, so clinics will choose a Day 5 to transfer first if they have the option.
But what happens if there are multiple embryos to choose from? What if your Day 5 embryo grades are not as good as your Day 6 embryo grades?
This is where grading and development timing come into play.
Day 5 vs. Day 6: Embryo Grades Matter
Sometimes, you may have embryos that reach the blastocyst stage on both Day 5 and Day 6. While Day 5 embryos are usually preferred, a high-grade Day 6 embryo can still be a great option. Here’s an example Alease shared:
“If we have a Day 5 3BC and a Day 6 4AA, the choice isn’t always clear-cut. In these cases, embryologists may consult with team members or the lab director to decide whether the timing (Day 5 vs. Day 6) or the quality (grade) should take precedence.”
For reference, embryo grades reflect the quality of the embryo, which looks at the expansion, the quality of the inner cell mass and the uniformity of the cells that will become the placenta, known as the trophectoderm. Higher grades, like 4AA, indicate well-structured embryos.
We’ve done an entire series all about embryo grades, which you can check out to learn more about what each grade means. We’ve even done a video series with Alease explaining embryo grading!
Are Day 7 Embryos Good?
Day 7 embryos are usually the last to be chosen for transfer. Research indicates that embryos reaching the blastocyst stage by Day 7 have the lowest chance of resulting in pregnancy. If a patient has Day 5 or Day 6 embryos available, those are prioritized over Day 7 embryos.
PGT Test Results Make a Difference
If you opt to have your embryos genetically tested by preimplantation genetic testing (PGT), the selection process becomes even more clear. If you do this testing, you know which embryos are genetically normal, meaning they have the correct amount of DNA in each cell.
If embryos don’t have the correct number of chromosomes in their cells, they will usually fail to implant or lead to genetic disorders such as Down Syndrome, in which cells have an extra 21 chromosome.
In these cases, embryologists will prioritize embryos that are genetically normal, ensuring the highest likelihood of implantation and a healthy pregnancy. This test trumps the embryo grade. Embryo grades are often used as a proxy to assess which embryo is most likely to be genetically normal. When you have the PGT results, this helps make it more clear which embryo is most likely to result in a healthy baby.
In Short: How to Choose Which Embryo To Transfer
In summary, these are the points embryologists look at when selecting an embryo during an IVF transfer cycle:
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Day 5 embryos are typically the top choice for transfer.
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Day 6 embryos may be selected if their quality surpasses that of available Day 5 embryos.
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Day 7 embryos are chosen last due to lower success rates.
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Higher graded embryos will likely be selected after that Day for development is taken into account
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If embryos are PGT testing, genetically normal embryos take priority before Day of development or grade.
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Priority List for How to Choose An Embryo to Transfer in IVF
1. PGT Results First: Genetically normal embryos
2. Day of Development: Day 5 is preferred over Day 6
3. Embryo Grade: Embryo grade is taken into account last with the highest grade being chosen
Choosing the right embryo is a collaborative process, and your embryology team will work to select the embryo with the best potential for success. Trust in their expertise, and know that every decision is made with the goal of helping you achieve a healthy pregnancy.
