I get hundreds of questions from viewers like you every week.
I get a lot of questions from people who want to understand how an embryo develops, what
the different stages are, why embryos sometimes don’t develop and how to use that information
to maximize their chances for IVF success.
I am going to make it really easy to understand.
Be sure to watch all the way to the end for my InfertilityTV Bottom Line.
Embryo development can be divided into various stages.
These stages are determined by looking at the embryos under a microscope.
Understanding embryo development can help improve your chances for pregnancy.
When the expert embryologists at my center, IVF1, inject a sperm into a mature egg, it
is not yet considered fertilized even though there is a sperm inside the egg.
After the sperm enters the egg, the egg is responsible for starting to organize all the
genetic material.
There are many steps to this process, but one step can be seen easily under the microscope.
This is when a new membrane forms around the chromosomes that came from the sperm and the egg.
These are called pronuclei..
A normally fertilized egg contains two pronuclei - one from the male and one from the female.
Pronuclei are abbreviated as PN.
You might hear your doctor or embryologist refer to a fertilized egg as a 2PN.
When you hear us refer to the fertilization rate, that means the percentage of eggs that
show two pronuclei after being injected with sperm.
Typically, about 70-80% of the eggs which are injected with sperm will become two pronuclei embryos
After fertilization, a healthy embryo will start dividing.
Healthy embryos will divide symmetrically.
This means a 1 cell embryo will divide into 2 cells, 2 cells divide into 4 cells and so on.
This is called the cleavage stage.
We know that some fertilized eggs will not divide at all.
Some will divide for a short time and then stop.
Other times, embryos will fragment or divide unevenly.
Embryos that stop developing are said to have arrested.
These embryos cannot produce a viable pregnancy.
Generally, a healthy embryo will divide into 8 cells by about the 3rd day after fertilization.
A large number of embryos don't develop beyond the 8 cell stage. Why not?
The answer is very important to understand.
In order to grow and develop, the fertilized egg needs proteins that originated in the
genes from the egg.
Let me put that another way.
Up until the 8 cell stage, or about 3 days, the sperm does nothing to contribute to embryo
development.
If your embryos stop developing before the 8 cell stage, that is an egg problem 100%
of the time.
In order to keep developing beyond the 8 cell stage, the embryo has to go through an important
change called “Embryonic Activation”.
You can’t see this under the microscope but this is the point that new genes are getting
turned on in the embryo.
These new genes come from both the egg and the sperm.
The new proteins allow the embryo to continue developing.
This is why world class IVF centers like mine don't do embryo transfers after only 3 days
Even if an embryo looks like a perfect 8 cell embryo under the microscope, you can't tell
whether it will undergo embryonic activation.
After you put it into the uterus, you can't see it all.
Unfortunately, many of those embryos don’t continue to develop.
In fact, if you look at the percentage of embryos that produce a pregnancy, Day 3,
8 cell embryos have a significantly lower chance compared to later developed embryos.
Getting pregnant with IVF is all about trying to predict which embryos are the healthy viable embryos
Think of it like this.
If you were trying to predict the results of a race in which the runners go six laps
around a track.
You would be more likely to pick the winner by looking at who was ahead after 5 laps compared
to who was ahead after three laps.
After the 8 cell stage, the cells of the dividing embryo start to fuse together.
The embryo is now called a morulae.
It looks kind of like a raspberry.
Under the microscope, the morulae stage embryo starts to produce fluid in the middle.
This is the beginning of what is known as the blastocyst stage.
Embryos may reach the blastocyst stage by the 5th to 6th day after fertilization.
As the fluid inside the embryo accumulates, the embryo grows larger, like a water balloon.
This is called an expanded blastocyst.
Many embryos never reach the blastocyst stage.
If an embryo stops developing before the 8 cell stage, it's an egg problem.
If it stops developing after the 8 cell stage, it could be due to problems with the egg,
problems with the sperm or both!
There is no easy way to tell.
However, the most common reason why embryos don’t reach the blastocyst stage is because
they have too many or too few chromosomes.
Culturing an embryo in the laboratory to the blastocyst stage is a pretty good way to screen
out some chromosomally abnormal embryos without having to do any testing on them..
Its not a perfect method however.
Some beautiful expanded blastocysts can still have chromosome abnormalities.
Our InfertilityTV Bottom line is this.
There are several stages of embryo development.
By looking at how embryos develop and when they sometimes stop developing, you can learn
about the causes of those problems.
Hi!
Doctor Morris here
I’m an IVF expert in the United States, near Chicago and the host of Youtube's most
popular and informative channel about trying to conceive - InfertilityTV.
If you want to improve your odds for success, hit that subscribe button right now.
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