One Embryo Transfer vs Your Overall Chance of IVF Success
When patients look at IVF success rates, it is very easy to focus on a single percentage without necessarily knowing what that percentage represents. Is it the chance of pregnancy from one embryo transfer? Is it the live birth rate per transfer, per egg collection or per treatment cycle? Does it include frozen embryos that may be transferred months or even years later?
These distinctions matter because, for most people, the question they are really asking is not, “What is the chance that this particular embryo transfer will work?” They want to know something much more personal and important: “What is my overall chance of having a baby from IVF?”
Those are not quite the same question.
An embryo transfer represents one opportunity for pregnancy, while an egg collection may create several embryos and therefore several opportunities over time. Understanding that difference can completely change the way patients interpret both their own IVF journey and the success rates they see published by fertility clinics.
Looking beyond the first embryo transfer
Consider a patient who has an egg collection and creates three embryos suitable for transfer. The first embryo is transferred and does not result in pregnancy. A second embryo is transferred later, again without success, while the third embryo ultimately results in the birth of a baby.
Looking only at the first transfer would give us one picture of that treatment. Looking at what ultimately happened with all the embryos created from the egg collection gives us another, and for the patient, the second picture is usually the one that matters most.
This is the thinking behind cumulative IVF success. Rather than considering each transfer completely in isolation, cumulative success looks at the possibility of achieving a live birth from the embryos created from an egg collection, including subsequent frozen embryo transfers within the period being measured.
It also explains why an unsuccessful embryo transfer, while understandably disappointing, does not necessarily tell us that an egg collection has been unsuccessful. If embryos remain frozen, the opportunity created by that collection has not necessarily been exhausted.
Why the IVF funnel matters here
Earlier in this series, we looked at embryo development and the IVF funnel: follicles become eggs, some eggs are mature, some fertilise normally and some of those fertilised eggs develop into blastocysts suitable for transfer or freezing.
It is natural to see the decreasing numbers through that process as a loss at each stage, but there is another way to think about the final number. Once suitable embryos have been created, each may represent another opportunity to achieve a pregnancy without repeating ovarian stimulation and egg collection.
This is also why the number of eggs collected should never be interpreted on its own. Collecting a larger number of eggs can provide more opportunities to create embryos, but what happens to those eggs afterwards is critical. Their maturity, fertilisation and subsequent embryo development ultimately determine how many opportunities for transfer have actually been created.
Two patients can therefore collect the same number of eggs and finish their cycles in very different positions. One may have a single embryo available for transfer, while another may have several. Even then, we cannot know with certainty which embryo will implant and continue to a healthy pregnancy.
There is probability throughout IVF, but very little certainty.
Age remains an important part of the picture
Age continues to have a significant influence when we talk about both individual transfers and overall IVF success because the likelihood of chromosomal abnormalities in eggs increases as women get older. This affects the chance that an egg will fertilise, develop into a viable embryo and ultimately result in a healthy pregnancy and baby.
There is an important point here for patients who have frozen embryos. An embryo does not continue ageing while it is frozen. When we consider the reproductive potential associated with the egg, the age of the woman — or egg provider — at the time the egg was collected is particularly relevant.
A woman may therefore return several years later to transfer an embryo created during an earlier IVF cycle. Although she is older at the time of transfer, the embryo was created from an egg collected at her younger age. Her current health and other factors still matter, of course, but the age of the egg has not changed while the embryo has been frozen.
This is one of the reasons IVF success needs to be discussed in context rather than reduced to a clinic-wide percentage.
What does cumulative live birth rate actually mean?
You may come across the term cumulative live birth rate when researching IVF success. Broadly, it describes the likelihood of achieving a live birth when outcomes from embryos created during a treatment cycle are considered over more than one transfer.
It can be a useful way of looking at IVF because it gets closer to the question many patients are really interested in: what might ultimately come from this treatment rather than what is the chance from one transfer?
However, even cumulative statistics need to be read carefully. Different reports and studies can define treatment cycles, time periods and included transfers differently, so two percentages that appear to describe the same thing may not have been calculated in exactly the same way.
The same applies to many IVF statistics. Pregnancy rate, clinical pregnancy rate, ongoing pregnancy rate and live birth rate describe different outcomes, while results reported per embryo transfer, per egg collection or per cycle started use different starting points.
This is where IVF success rates can become confusing, particularly when they are presented as a single large percentage without enough explanation around them.
A success rate should help you understand IVF, not sell it to you
At No.1 Fertility, we think patients deserve to know what sits behind a success rate. A percentage can be useful, but only when you understand who it relates to, what outcome is being measured and at what point in treatment the calculation begins.
Age is one of the strongest influences on IVF success, but it is not the only consideration. Ovarian reserve, sperm factors, previous pregnancy and treatment history, the number of eggs collected and, eventually, the number and development of embryos all contribute to the clinical picture.
Importantly, our understanding of an individual patient’s situation also develops as treatment progresses. Before an IVF cycle begins, we are working with medical history, investigations and probabilities. After egg collection and embryo culture, we know more. We have seen how the ovaries responded, how many eggs were mature, what happened at fertilisation and how the embryos developed.
If an embryo transfer does not result in pregnancy, that information becomes part of the picture as well. If frozen embryos remain, they remain part of the opportunity created by that egg collection.
This is why our doctors and scientists look at an IVF cycle as a whole rather than reducing it to whether one transfer was positive or negative.
What does this mean for your own chance of success?
When discussing IVF with your fertility specialist, asking “What is the success rate?” is a reasonable place to start, but it should not be where the conversation ends. A more useful discussion is about what success rates mean for someone of your age and circumstances, what you might reasonably expect from an egg collection and, if embryos are created, what those embryos could mean for your overall chance of achieving a live birth.
There will never be a percentage that can predict exactly what will happen to one individual. IVF statistics describe what has happened across groups of patients; they cannot tell us with certainty what will happen to the person sitting in front of us.
What they can do, when explained properly, is help patients make more informed decisions and understand where they are in their treatment.
One embryo transfer is an important moment, but it is not always the whole measure of an IVF cycle. If an egg collection creates several embryos, the opportunity created by that treatment can extend beyond the first transfer and sometimes across several attempts.
For most patients, that broader picture is what IVF success really means: not the result of one transfer in isolation, but the overall opportunity treatment has created to achieve the outcome everyone is working towards — a healthy baby.
Understand your IVF journey in context.
Speak with our fertility specialists about what success rates may mean for you.