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IVF Cycle Embryo Development Day-by-Day - What are your embryos up to?

What stage are your embryos at? And what is normal development for each day? This is what your eggs & subsequent embryos are up to in the week after your egg retrieval for your IVF cycle.
Waiting after your egg retrieval for fertilization & embryo development results can be stressful. Image of clock lying on a surface which is half baby blue and half baby pink.
Waiting after your egg retrieval for fertilization & embryo development results can be stressful. Image of clock lying on a surface which is half baby blue and half baby pink.
After your egg retrieval is complete, you'll be spending the next week waiting for updates from the embryology lab at your fertility clinic on how your IVF cycle is going.

During the start of an IVF cycle leading up to the egg retrieval, it can feel like a whirlwind of scheduling, injections, ultrasound & bloodwork appointments, and countless phone calls.

And then all of a sudden after the egg retrieval, your part is done. And there's nothing to do but wait for updates on how the cycle went.

It can be a little nerve-wracking waiting for news about your embryos if you feel like you don't have any insight into the process.

What happened to my eggs after the retrieval? How many eggs did I get? Did they fertilize? Did any embryos develop? How many embryos will I end up with?

Here's the BTS (behind the scenes 😉) of the embryology lab for what stage your embryos should be at each of the days of development.

Embryo Development Timeline for IVF

 

Day 0️⃣


This is how we count in the embryology lab. We start from Day 0, which makes it easier to count up the days of embryo development.

This is the day of the:
👉egg retrieval
👉egg fertilization attempt (ICSI/conventional IVF)
 
The first thing you'll probably be thinking about is whether your eggs were successfully retrieved and how many you got.

According to the most recent data from the CDC, 91.3% of egg retrievals were successful in recovering eggs. Those are some good odds, however this rate lowers as a woman's age increases.

Insemination


After your egg retrieval, your eggs are counted, cleaned (if you're doing ICSI), and inseminated.

For ICSI, an embryologist will inject a single sperm into each egg.

For standard IVF, the eggs will be placed in a dish with the sperm so that a single sperm can hopefully reach an egg on its own and successfully fertilize it.

The eggs are stored overnight in an incubator with the correct gas mixture, temperature, & humidity needed to simulate being inside the body.

The next day, an embryologist will check each egg for signs of successful fertilization! 🙌

Oocytes are normally checked for signs of fertilization about 16-18 hours post-insemination.
 

Day 1️⃣


Time to check for successful fertilization!

Remember, only mature eggs have the potential to be fertilized.

Because of this, your embryology team will keep track of, not only how many eggs were retrieved, but how many were mature and could be used for an insemination attempt.

You'll find out today how many eggs were:
👉retrieved
👉mature
👉fertilized

Egg Maturity


Keep in mind that your team will only know how many eggs were mature if you did ICSI. Why?

When eggs are retrieved, they are covered in a sticky coating of cells called cumulus cells.

Eggs are 'cleaned' for ICSI, meaning that this cellular coating is removed from the surface of each egg. This is done so that the embryologist can identify which eggs are mature and can be injected with a sperm.

In standard IVF, where the sperm are place with the eggs in a dish overnight, they don't clean the eggs until the next day when they check fo fertilization. So they won't know how many eggs were mature on the day of insemination.

Fertilization


How do they know which of your eggs fertilized? 🤷‍♀️
 
The day after IVF/ICSI, all eggs are checked for signs of fertilization. Successful fertilization is identified by the presence of 2 pronuclei. Zygote image: ESHRE Atlas of Human Embryology.
The day after IVF/ICSI, all eggs are checked for signs of fertilization. Successful fertilization is identified by the presence of 2 pronuclei. Zygote image: ESHRE Atlas of Human Embryology.


The embryologists are looking for ‘2PN 2PB’, which is short for '2 pronuclei and 2 polar bodies'. The primary focus is on the number of PN they see under the microscope.
 
2PN = normal fertilization
 
🔬The two pronuclei (2PN) look like two bubbles floating on top of the egg with little granules inside of each. This is the DNA!

If your clinic shares time-lapse footage of your embryos, you may see the two pronuclei pop out at the start of the time-lapse & then disappear back into the embryo before it starts dividing.

At any given stage of development, some embryos may not divide normally & arrest (stop developing) but others continue on! In some cases, no embryos will make it to the stage where they can be transferred or frozen.

Of women that successfully retrieve eggs, the CDC reported that 87.5% of cycles successfully continue on to an embryo transfer or freezing of embryos.
 

Day 3️⃣


Your embryos should have been dividing over the past couple of days and reached the ~8-cell stage by now.

Each embryo is graded at this point to see whether they measure up to the typical embryo development timeline for a Day 3 embryo.

The cells are checked for number, equal-sizing, nuclei, and fragmentation.

✅An embryo with 8-10 equal-sized cells is considered to be right on track.

This is an example of a good quality Day 3 embryo. It has 8 cells, cells of equal size, & no fragmentation. Photo Credit: ESHRE Atlas of Human Embryology
This is an example of a good quality Day 3 embryo. It has 8 cells, cells of equal size, & no fragmentation. Photo Credit: ESHRE Atlas of Human Embryology
 
Some patients do a Day 3 embryo transfer at this time.

However, it’s not the most common time to transfer in an IVF cycle. 
 
From this point, embryos continue dividing into a bundle of cells, known as a morula.
 

Day 5️⃣ / 6️⃣


This is when you start to see embryos at the most well-known stage in IVF. The blastocyst stage.
 
Embryos often develop at different rates from one another. Some embryos reach the blastocyst stage by Day 5 while others take until Day to form.

Blastocysts are very easy to identify, as they have a very distinctive cell architecture to them.

At this point, you can now see the Inner Cell Mass, which are the cells that become the baby, and the surrounding Trophectoderm cells, which form the placenta upon implantation.

Inner Cell Mass = Baby 👶
Trophectoderm = Placenta
 
The trophectoderm is actually what is tested for Preimplantation Genetic Testing.

The anatomy of a blastocyst. An embryo reaches the blastocyst stage around Day 5 or 6 of development during an IVF cycle. At this time, it can be transferred, biopsied for PGT-A/PGS, and/or frozen for a future FET. Embryo image: ESHRE Atlas of Human Embryology.
The anatomy of a blastocyst. An embryo reaches the blastocyst stage around Day 5 or 6 of development during an IVF cycle. At this time, it can be transferred, biopsied for PGT-A/PGS, and/or frozen for a future FET. Embryo image: ESHRE Atlas of Human Embryology.


So if you're doing genetic testing such as PGT-A or PGT-M or PGT-SR, it might be handy to know that the baby's cells are not tested directly. Just a few of the embryo's pre-placental cells are tested.

If you want to learn more about the behind the scenes of PGT-A, check out our guide to preimplantation genetic testing.

In that article, we talk about what PGT-A results can & cannot tell you, what happens to your embryos, the cost, test accuracy, and more. 

Blastocyst Embryo Grading


At this point, the embryos are graded again. This is the stage where you start to see a grading system such as 4AB, 5AA, 4BC, etc.

The embryologists use criteria to assess which embryos have developed the most optimally.

They give a grade for each of these three different factors:
  1. the embryo expansion
  2. the inner cell mass
  3. the trophectoderm

Not all clinics use the same blastocyst grading scale but the common scale is:
👉expansion graded 1-6 (6 = optimal)
👉inner cell mass graded A-C (A = optimal)
👉trophectoderm graded A-C (A = optimal)

For example, an embryo with an a good grade would be a 5AA embryo.

An embryo with a poor grade would be 2BC.

While studies have shown that there is an association between an embryo with a poor grade having lower pregnancy success rates, as they more often do not have the correct number of chromosomes, it doesn't mean that all poorly graded embryos result in a failed transfer!

Some embryos that develop slowly or have a poor grading are chromosomally normal and do result in a healthy baby.

PGT-A


One way you would have more information ahead of the transfer would be to test with PGT-A to see whether your embryos have the correct number of chromosomes needed for life.

It's optional and some patients just choose to transfer the best graded embryos they have without testing.

A lot of the choice to test your embryo can depend on your age.

Older women have a higher likelihood of chromosomal abnormalities in their embryos, while younger women tend to have more embryos that are chromosomally normal.

If you're wondering whether the added cost & time of testing your embryos is worth it, check out out this recent study done assessing the cost benefits of doing PGT-A based on your age.

What happens once you have blastocysts?


⬇️Depending on your & your fertility doctor's plan, next steps might be:
 

⤴️Fresh transfer

Some women do fresh an embryo transfer now to attempt a pregnancy.
 

🧬Embryo Biopsy

Some patients may choose to test their embryos at this time by either PGT-A, PGT-M, or PGT-SR, depending on what they are trying to test for.
 
Steps for preimplantation genetic testing:
👉embryos are biopsied
👉biopsies are sent to a PGT lab
👉embryos are frozen to await results
 

❄️Embryo Freezing


Whether you're genetically testing or not, embryos can be frozen at this time & later thawed for a frozen embryo transfer cycle (FET).

A frozen embryo transfer is often done the next month after the IVF cycle. This is done often for women who may have had OHSS from their IVF cycle, so could not do a fresh transfer, or for women who did PGT-A that need to wait for the test results.

Even if you do a fresh transfer this cycle & have a successful pregnancy, you can freeze your remaining embryos on Day 5 or 6 so that if you'd like another child in the future, you hopefully won't have to do another egg retrieval and can instead have an FET.
 
We hope knowing what your embryos may be up to during this week helps while you wait for updates!

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