From Eggs to Blastocysts

From Eggs to Blastocysts to baby

What does from eggs to blastocysts actually mean? During IVF it is normal to see a continual drop in numbers in embryo development, which, can lead to the best embryos that will give you your baby..

Undoubtedly, one of the biggest surprises with in vitro fertilization (IVF) is the dramatic drop or loss of eggs/embryos. Many people assume that if they have 10 eggs collected, that’s 10 embryos, which means 1 fresh transfer and 9 in the freezer.

Unfortunately, that can be far from true and there are multiple steps along the way that results can take an unexpected turn. This “numbers game” is what is known as IVF attrition. There are no set rules for this game, meaning that it’s impossible for a doctor to predict what may happen with a patient. In addition, results can vary from person to person.

So what are realistic expectations? Embryologist, Kristin Jones, shares what is generally known about IVF attrition.

Attrition With Egg Retrieval

You may not get as many eggs collected as you expect:

  • Some follicles stimulate that have nothing in them.
  • Sometimes when the doctor goes in and looks around, some of the follicles just aren’t there anymore.
  • Sometimes at egg collection, the fluid assessed, has large masses of cumulus cells (the support cells that surround an egg) that do not contain an egg inside. Even though there is no egg, it may lead to a growing follicle and be an expected egg.

In an IVF cycle, medication is used to hyper-stimulate ovaries to develop multiple follicles that hopefully contain eggs. Constant monitoring with ultrasound is done and you should get an idea of how many eggs are expected to be collected based on the size of your follicles at the ultrasound and your estrogen levels.

So, please note: the number of follicles seen on ultrasound does not necessarily equal the number of eggs that will be retrieved. Attrition in the IVF process happens right from the start.

Attrition During Fertilization

After the egg collection, approximately 4-6 hours later, the eggs get inseminated with sperm. This can happen with either:

Your specialist will make the decision of which one is best for your situation.

Mature Eggs Only

Not all eggs that are retrieved are able to be fertilized. Eggs start off with 46 chromosomes and need to undergo a process of shedding extra chromosomes to become a mature egg. For fertilization to properly occur, a healthy egg cell should have 23 chromosomes that can pair with the 23 chromosomes from a sperm cell.

If ICSI is being performed, only the mature eggs can be injected and it’s common to have some eggs that are not.

Overall, it’s expected that about 80% of eggs should be mature, but just as with every step, this can vary greatly among patients—and even subsequent cycles.

Kristen commonly sees that patients have all mature eggs, but it’s very common to see some immature ones in there as well. Very rarely, they can all be immature!

If IVF is being done, all eggs are going to get inseminated (as eggs are left in their fluffy cumulus cells) and the next morning we will see which ones are fertilized. Again, the immature ones will not fertilize.

Fertilization Rates

With 12 eggs collected, 10 can be injected with ICSI, or only 10 will be able to be fertilized by the sperm with IVF. Of those mature eggs that are capable of fertilizing, they are not all expected to fertilize. Approximately 70-80% is a good fertilization rate but many labs are happy with less than this as they may treat poorer prognosis patients that aren’t expected to do as well.

Absolutely, many patients can get 100% fertilization, but it’s important to know that seeing some not fertilize is completely normal. Kristen may also see some that fertilize abnormally and need to be discarded. With this being said, patients who had only a small fraction of their eggs fertilize, will get pregnant from those few embryos.

Attrition During Embryo to Blastocyst Development

Most fertilized eggs, or embryos as they now are, will divide onto a day 3 embryo; as the cells divide, the quality may start to deteriorate, cell fragment, and may divide unevenly. For this reason, not all embryos will be suitable for transfer or freeze on day 3 if your doctor/clinic has chosen day 3 for you.

While this may be disappointing, please know that it’s this process that allows Kristen and her team to choose the ones that are best for use and allows some genetically abnormal embryos to deselect themselves from the batch. These embryos would not have been capable of forming a healthy pregnancy.

The Day 3 Drop

A big drop in numbers usually occurs in the phase between day 3 and day 5 of embryo development. Not all embryos will be able to form a blastocyst and this is because the egg has everything in it to drive growth to get to day 3 and then embryonic genome activation (EGA) has to occur.

This means that embryo development is under the control of the maternal egg genes until day 3 and then control is taken over by the genes of the zygote which many embryos just can’t do and commonly is because of a chromosomal problem. It’s an advantage to grow to day 5 as only the developmentally competent embryos are able to reach blastocyst.

A day 3 embryo is a little bundle of 6-10 cells and there are major growth and structural changes that need to happen to reach the stage of a 100-150 cell blastocyst, so the poorer quality embryos may arrest, or stop developing at this stage.

It is also possible that embryos make it to blastocyst but are just too poor quality to warrant a transfer or freeze. A lot of patients do not realize that this is a possibility.

Blastocyst Success Rates

You may see it written that 30-50% of day 3 embryos might be expected to reach blastocyst. Keep in mind that this is going to differ from person to person and is very hard to provide specific stats on due to the many variations and factors at play from age of the egg used to the genetics in each sex cell and how they combine.

It’s not just numbers but the quality that is important too. I’ve seen patients with over 12 eggs and very good fertilization not have anything suitable for transfer or freeze due to poor quality.

Receiving Unexpected Results

If you have been faced with any of these unexpected results, please know that this situation has been faced by many people before you and you are not alone. Having a poor cycle doesn’t necessarily mean you will get the same results again. There are different medical protocols or lab options (such as different day embryo transfer, fresh/frozen, IVF/ICSI) that may lead to you getting different results in a subsequent cycle.

From Kristen’s experience, it’s common to be shocked at how an IVF cycle plays out, so please know, it is normal to see a continual drop in numbers and is what hopefully leads embryologists to the few best embryos that will give you your baby.

Egg Donation, Inc’s Southern California agency in Los Angeles is proud to work alongside some of the best IVF Clinics throughout the Nation. At the forefront of advances in egg donation technology, including egg banking of frozen donor eggs, we invite you to see why we are one of the leading egg donation agencies. Visit our about us pages to see what makes us the agency of choice.

Story Source: This article originally appeared on Fertility Smart’s website. Written by: Kristen Jones (Accredited Embryologist at I Like My Eggs Fertilised )