Skip to:
One of the most emotionally difficult parts of IVF is discovering that the number of eggs collected does not equal the number of embryos created.
Patients are often told they have “10 eggs” or “15 eggs,” only to later hear that only a handful are fertilised, fewer continue developing, and sometimes none reach the blastocyst stage. For many people, this can be confusing, and deeply distressing, especially when the natural drop-off that occurs throughout IVF is not clearly explained before treatment begins.
The reality is that IVF is a process of biological selection. At every stage, eggs and embryos are assessed by nature and by the laboratory. Not every egg is mature, not every mature egg fertilises, and not every fertilised egg has the genetic or cellular capability to continue developing into a healthy embryo. Similar problems around genetic or cellular capabilities can affect sperm as well as contribute to the drop off.
Understanding this “IVF attrition” or “IVF funnel” can help patients approach treatment with more realistic expectations and less self-blame.
During IVF, there are several key developmental stages between egg collection and embryo creation:
At each step, numbers naturally decline.
For example, if 10 eggs are collected, it is biologically normal that:
The Fertility Society of Australia and New Zealand (FSANZ) notes that IVF outcomes depend on many interacting biological factors, from age, egg quality and sperm quality to embryo development.
One of the most commonly searched IVF questions is:
“Why did only some of my eggs fertilise?”
Fertilisation requires both a healthy mature egg and viable sperm. Even with IVF or ICSI, fertilisation is not guaranteed.
Common reasons eggs may not fertilise include:
Even when sperm is injected directly into the egg through intra-cytoplasmic sperm injections (ICSI), the egg still needs to activate and begin embryo development on its own. Some eggs simply do not have the capacity to do this. In addition the sperm need to be genetically competent.
Research and clinical data consistently show that attrition between egg collection and fertilisation is expected in IVF treatment.
Another common question patients ask is:
“Why did my embryos arrest?”
Embryo arrest, which is an unfortunate term, means an embryo has stopped developing before reaching the blastocyst stage.
This can happen for many reasons, including:
Importantly, embryo arrest is often not caused by errors on the patient or clinic’s behalf, but rather external factors.
In fact, embryo development is heavily influenced by chromosomal normality. Human reproduction is biologically inefficient, and many embryos created naturally, not just in IVF, would never progress to pregnancy.
Laboratory monitoring allows embryologists to observe this process in real time, which is why IVF can sometimes feel confronting. Processes that would normally happen unseen inside the body become visible more during treatment.
Not necessarily. While a higher number of eggs may increase the chance of creating embryos overall, egg quality is often more important than egg quantity.
Age remains one of the strongest predictors of IVF success because egg quality and chromosomal normality decline over time. Australian and New Zealand ART data shows live birth rates reduce significantly with advancing maternal age.
Some patients with fewer eggs may still create healthy embryos, while others with high egg numbers may experience significant attrition.
This is why fertility specialists often focus on:
rather than egg numbers alone.
There is no universal percentage because IVF outcomes vary from person to person and treatment cycle to treatment cycle. Success rates are influenced by factors such as age, ovarian reserve, sperm quality, underlying fertility conditions, response to stimulation, and laboratory processes.
However, many fertility clinics counsel patients that approximately:
These are broad estimates only and should not be interpreted as guarantees.
Under Australian guidelines and AHPRA advertising standards, IVF success information must be presented carefully and without misleading claims or unrealistic expectations.
One of the hardest aspects of IVF is the emotional impact of expectation versus reality.
Patients often begin treatment hopeful after hearing promising egg numbers, only to feel devastated by the attrition that follows. Yet biologically, this drop-off is entirely natural, anticipated and necessary for finding “ good eggs”.
More transparent education around embryo development can help reduce unnecessary guilt, confusion and self-blame during fertility treatment.
IVF is not simply a numbers game. It is a process of identifying which eggs and embryos have the developmental potential to continue toward pregnancy.
While attrition can feel discouraging, it is an important part of how embryology works to identify the embryos most likely to result in a healthy pregnancy and a happy baby.
It is important to discuss your individual cycle outcomes with your fertility specialist, particularly if you have concerns.
In some cases, additional investigations or changes to treatment protocols may be recommended.
Every IVF cycle provides valuable clinical information, and treatment decisions should always be personalised to your circumstances.
For evidence-based fertility care and personalised IVF guidance, visit Dr Rachael Knight at Stellar Fertility.
