For many patients, the IVF laboratory is the biggest mystery of the entire treatment process.
What exactly happens to the eggs after the retrieval? How are embryos created? How do embryologists know which embryo has the greatest potential? And what does "blastocyst" actually mean?
To help answer these questions, Andrea Nesvadbová, Head of the Embryology Laboratory at Unica Clinic Brno, takes you behind the scenes of one of the most important parts of IVF treatment.
🎥 Watch the full video below to see how embryos develop in our laboratory.
Every IVF journey begins with a single egg
The laboratory journey starts on the day of egg retrieval.
After the eggs are collected, they are transferred to special incubators that recreate the conditions of the female reproductive system as closely as possible.
"The incubator replaces the woman's reproductive system. It provides the environment embryos need to develop during the first days of life," explains Andrea Nesvadbová.
Before fertilization, embryologists carefully examine every retrieved egg.
Not every retrieved egg is ready for fertilization
Although hormonal stimulation allows several eggs to mature during one cycle, not every retrieved egg is fully mature.
The first task of the embryologist is to gently remove the surrounding cumulus cells. Only then can they determine whether an egg has reached the stage where fertilization is possible.
"After removing the surrounding cells, we can determine whether the egg is mature and ready for fertilization," Andrea explains.
Only mature eggs continue to the next stage of treatment.
How does fertilization happen in the laboratory?
Unlike natural conception, where one sperm fertilizes the egg inside the fallopian tube, IVF allows embryologists to carefully select a single sperm with the best movement and appearance.
That sperm is then injected directly into the mature egg using a highly precise technique called ICSI (Intracytoplasmic Sperm Injection).
Before fertilization, sperm undergo specialized laboratory preparation using advanced selection methods that help embryologists isolate the healthiest and most motile sperm cells available.
The first important milestone: Has fertilization occurred?
Approximately 17 hours later, embryologists perform the first assessment.
At this point they look for two pronuclei, one from the egg and one from the sperm, which confirms that fertilization has successfully taken place.
"The presence of two pronuclei tells us that fertilization has occurred," Andrea says.
Watching an embryo grow
Over the following five to six days, the embryo continues dividing.
From two cells...
...to four...
...to eight...
...until it reaches the blastocyst stage, containing around 70–250 cells.
At Unica Clinic, embryos are usually cultured until Day 5 or Day 6 because this provides embryologists with much more information about their developmental potential.
"We recommend culturing embryos until the blastocyst stage, when we can better evaluate their development," Andrea explains.
A blastocyst is the stage at which the embryo has developed into two distinct cell groups.
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One group, called the trophectoderm, will later form the placenta.
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The second group, known as the inner cell mass, will eventually become the baby itself.
Both structures are carefully evaluated before embryologists recommend an embryo for transfer or freezing.
How do embryologists evaluate embryo quality?
Once an embryo reaches the blastocyst stage, embryologists assess it using an internationally recognized grading system.
Embryos receive grades such as AA, AB, BA or BB, based on the appearance of the inner cell mass and the trophectoderm.
AA embryos are generally considered to have the highest developmental potential.
However, Andrea points out an important fact many patients don't know.
"Even the most beautiful embryo does not guarantee pregnancy."
Why? Because embryo appearance tells only part of the story.
Why can a perfect-looking embryo still fail?
Under the microscope, embryologists can evaluate how an embryo develops and whether it reaches the blastocyst stage.
What they cannot see are its chromosomes.
Scientific studies show that many eggs naturally contain chromosomal abnormalities, particularly as maternal age increases. This means an embryo may look excellent morphologically but still carry genetic abnormalities that prevent implantation or lead to an early miscarriage.
"Morphology shows us how an embryo develops, but it cannot tell us whether its chromosomes are normal," Andrea explains.
Behind every embryo is a dedicated team
Every embryo is unique.
Behind each one stands a team of experienced embryologists making hundreds of precise decisions throughout its development—from selecting mature eggs and sperm to evaluating embryo quality and determining the best time for transfer.
At Unica Clinic, advanced laboratory technology always goes hand in hand with the expertise and careful judgement of our embryologists.
If you'd like to learn more about our IVF laboratory or discuss your treatment options, our team is here to answer your questions and help you take the next step toward parenthood.
