Embryo Development and IVF Success
When people talk about an IVF cycle, one of the first questions is often, “How many eggs did you get?”
It is an understandable question. Egg collection is the first time during treatment that there is a tangible number to hold onto, and it can feel as though more eggs must automatically mean a greater chance of success.
But the number collected is only the beginning of the story.
Over the next five or six days, we begin to learn much more. We see how many eggs are mature, how many fertilise normally and, importantly, how those fertilised eggs continue to develop. Some embryos will progress steadily towards the blastocyst stage, while others will slow or stop developing along the way.
This is sometimes described as the IVF funnel. The numbers generally become smaller as a cycle progresses, but that does not necessarily mean something has gone wrong. It reflects something fundamental about human reproduction: not every egg has the capacity to become an embryo and not every embryo has the biological potential to become a healthy pregnancy.
IVF simply allows us to see much more of that process.
What happens after egg collection?
Once eggs arrive in the laboratory, what happens next depends partly on the fertilisation method being used.
If the eggs are intended for ICSI, where a single sperm is injected directly into the egg, our scientists first remove the support cells surrounding each egg so they can assess its maturity. Only mature eggs are suitable for ICSI.
If the eggs are being fertilised by natural insemination, often referred to as conventional IVF, those surrounding cells remain in place. In that situation, the maturity of each egg is not known at the time of insemination and is usually only evident the following day, after the sperm have helped remove the surrounding cells.
The following day, we look for signs of normal fertilisation. From there, the embryos that have fertilised begin dividing and developing.
During the first few days, an embryo progresses from a single fertilised cell through successive cell divisions. By approximately day five or six, some will reach the blastocyst stage. At this point, the embryo has developed into a much more complex structure containing cells that will eventually contribute to the baby and the placenta.
Reaching blastocyst is an important milestone because an embryo may then be suitable for transfer, freezing or, when clinically indicated, genetic testing.
But there is a natural attrition between egg collection and blastocyst formation. Understanding that before treatment can make the numbers encountered during an IVF cycle much easier to interpret.
Why do embryos sometimes stop developing?
This is one of the most difficult questions in IVF because there is not always a single answer.
Embryo development depends on both the egg and sperm and on the genetic and cellular processes that begin immediately after fertilisation. Some embryos stop developing because they have chromosomal abnormalities or other biological problems that prevent them from progressing normally.
Age is an important part of this picture because the likelihood of chromosomal abnormalities in eggs increases as women get older. This helps explain something patients sometimes find confusing: it may still be possible to collect several eggs at an older reproductive age, yet the chance of each individual egg ultimately producing a chromosomally normal embryo is lower.
Sperm contributes half of the embryo's genetic material as well. Fertilisation and embryo development therefore need to be considered in the context of both partners where appropriate.
Even with everything modern embryology can tell us, however, we cannot determine an embryo's entire potential simply by looking at it.
An embryo can appear to be developing beautifully and still have a chromosomal abnormality. Another may not look quite as perfect at a particular moment and still result in a healthy pregnancy.
Embryology gives us information. It does not give us certainty.
The laboratory cannot change an embryo's genetics — but it can protect its opportunity to develop
This distinction is important when discussing IVF success.
A laboratory cannot turn an abnormal embryo into a normal one, and technology cannot overcome every biological limitation. What an excellent laboratory can do is create a carefully controlled environment in which eggs and embryos are given the opportunity to develop with as little unnecessary disturbance as possible.
Embryos are sensitive to changes in temperature, pH and their surrounding environment. Incubation conditions, air quality, handling, monitoring and the experience of the scientists caring for them all matter.
At No.1 Fertility, this is one of the reasons we have invested so heavily in the laboratory itself.
Our time-lapse incubation technology allows embryo development to be observed over time while embryos remain within their controlled culture environment. Rather than relying only on individual snapshots taken at particular moments, our scientists can see more of the pattern of development across those crucial early days.
We also pay close attention to the environment surrounding the laboratory, the monitoring systems protecting it and the infrastructure supporting our equipment and cryostorage.
It is also why we show patients our laboratory during our clinic tours.
We believe patients should be able to understand where their eggs, sperm and embryos are cared for and meet some of the people responsible for that care. The laboratory is too important to IVF treatment to remain an invisible part of the process.
Is a blastocyst a successful embryo?
Reaching blastocyst is encouraging, but IVF success cannot be measured at that point alone.
A blastocyst still needs to implant, establish a pregnancy and continue developing. Where preimplantation genetic testing is clinically appropriate, testing may provide additional information about chromosomal status, but no single assessment can guarantee that an embryo will result in a baby.
This is why IVF results need to be considered as a series of connected stages rather than one headline number.
Imagine, for example, that ten eggs are collected. Some may be immature. Of those that are mature, some may not fertilise normally. Of those that fertilise, some may stop developing before blastocyst. The final number of embryos available for transfer or freezing may therefore be considerably smaller than the original number of eggs collected.
That reduction can feel disappointing when it happens, particularly if nobody has explained beforehand that some degree of attrition is expected.
The important question is not simply, “How many eggs did we collect?”
It is, “What did we learn as the cycle progressed?”
Every stage of an IVF cycle gives us information
This is where embryo development becomes particularly important.
If fewer eggs are mature than expected, that tells us something. If fertilisation is unexpectedly low, we look at what may have contributed. If embryos repeatedly stop developing, we consider the pattern alongside age, egg factors, sperm factors and the rest of the clinical picture. If good blastocysts are being created but pregnancy is not occurring, our attention may need to move further along the IVF journey towards implantation and other possible factors.
Sometimes there is a clear explanation. Sometimes there isn't.
What matters is that the information from a cycle is not discarded simply because it did not produce the outcome everyone hoped for.
At No.1 Fertility, our fertility specialists and scientists look at the cycle together. We consider ovarian response, egg numbers and maturity, fertilisation, sperm, embryo development, blastocyst formation, transfer and the outcome. Previous cycles — including treatment undertaken elsewhere — can also provide valuable information when we are trying to understand what might be done differently.
IVF should not simply be about repeating treatment. It should also be about learning from it.
So, what should you focus on?
It is very easy to become consumed by numbers during IVF. Follicles become eggs; eggs may be shown to be mature; mature eggs may fertilise; fertilised eggs become embryos; and eventually there may be one or more blastocysts available for transfer or freezing.
Every one of those numbers means something, but none tells the whole story by itself.
Two people of the same age can collect the same number of eggs and have very different outcomes. A larger number of eggs does not, on its own, tell us what the final outcome of a cycle will be. Ultimately, what matters is whether an embryo with the potential to establish a healthy pregnancy can be created.
That is why understanding embryo development is such an important part of understanding IVF success.
The days between egg collection and embryo transfer can sometimes feel like waiting. In the laboratory, however, an extraordinary amount is happening. Embryos are developing, scientists are observing, and with each stage we are learning more about the biology behind that particular IVF journey.
The goal was never simply to collect the most eggs or create the most embryos.
The goal is a healthy baby — and understanding what happens at each stage gives us a clearer picture of where that opportunity lies and what we can learn from each IVF cycle.