Ever wondered why IVF numbers drop from eggs to embryos — and what’s normal? In this session, Dr.Renata Huttelova, PhD (Head of Embryology Lab, IVF CUBE, Czechia) and David O’Rourke (IVF Lab Director, Freya Fertility, Denmark) explain the real IVF “funnel,” also called embryo attrition rate.
Hosted by Sandy Christiansen, MSc, Clinical Embryologist & Fertility Educator, this Q&A session helps patients set realistic expectations and understand where (and why) embryos are lost along the way.
Dr Renata Huttelova, PhD, Head of Embryology Lab, IVF CUBE, Czechia: I have over 30 years of experience in embryology. I currently lead the embryology laboratory in Prague, am President of the Czech Embryology Association, and conduct audits for IVF centres seeking certification and quality standards.
David O’Rourke, IVF Lab Director, Freya Fertility, Denmark: I am the Director of Embryology at Freya Fertility in Denmark. I have worked as an embryologist for approximately 15 years across several European countries, including the UK.
David O’Rourke, IVF Lab Director, Freya Fertility, Denmark: There is a significant range in the number of oocytes collected, and this is largely determined by the stimulation protocol chosen by the doctor. That decision is based on hormone testing and the patient’s prognosis, which then guides the medication dosage.
In general, we aim for around 10 to 15 mature oocytes. That is an overall average, and there are many patients who will have fewer and many who will have more. The key point is the number of mature oocytes, as that is the most important factor at egg collection. There is a window that we aim for, but individual variation is expected.
Dr Renata Huttelova, PhD, Head of Embryology Lab, IVF CUBE, Czechia: Yes, this aligns with our experience. We usually aim for around 10 to 12, up to 15 eggs. This range is considered safe for patients, and safety is always a priority. The stimulation protocol must be optimised for each individual patient, taking into account age and other relevant factors.
David O’Rourke, IVF Lab Director, Freya Fertility, Denmark: Female age is the most important prognostic factor. Patients aged 35 or younger fall into the best prognosis group. When the issue is anatomical, such as missing fallopian tubes, rather than hormonal, this is generally positive.
Without knowing specific hormone levels such as AMH, we can assume they are within a normal range. In that scenario, we would aim for the ideal range of 10 to 15 mature oocytes. This number would usually provide enough embryos to allow for a transfer and potentially embryos for cryopreservation. Based on age alone, this patient would fit within that expected window, and the doctor would tailor stimulation to achieve that outcome.
Dr Renata Huttelova, PhD, Head of Embryology Lab, IVF CUBE, Czechia: There can still be issues with sperm–egg interaction, even when oocytes are mature, so some mature oocytes may not fertilise. At the age of 40, egg quality can vary significantly, and maturity alone does not reflect overall quality. We assess additional indicators of egg quality beyond maturity.
Fertilisation itself is usually not the most complex step; embryo development after fertilisation is more informative. Without additional information, it is difficult to draw firm conclusions. However, obtaining 1 blastocyst from 2 fertilised oocytes at age 40 can be considered a good result.
David O’Rourke, IVF Lab Director, Freya Fertility, Denmark: On paper, 2 fertilised oocytes out of 7 mature oocytes is below what we would typically expect and would be considered suboptimal. This would usually prompt a follow-up consultation to review what may have contributed, such as the fertilisation method used, sperm factors, or stimulation protocol.
That said, achieving 1 blastocyst from 2 fertilised oocytes at age 40 is actually a very good outcome. It is also important to consider the chromosomal status of the blastocyst, particularly at this age, as aneuploidy rates increase significantly after 35. If PGT testing was performed, that information would be very relevant.
Dr Renata Huttelova, PhD, Head of Embryology Lab, IVF CUBE, Czechia: Each stimulation cycle can yield different results. Even if the same protocol is used, the body can respond each time differently, leading to variations in egg number and quality. This is important for patients to understand when considering another cycle.
David O’Rourke, IVF Lab Director, Freya Fertility, Denmark: Over the past decade, many clinics have moved towards blastocyst culture. Keeping embryos in the lab longer allows us to gather more developmental information, which helps with embryo selection and estimating the chance of live birth.
Day 3 transfers can and do result in healthy pregnancies. However, by day 5, embryos have undergone compaction and formed a blastocyst with two distinct cell types: the inner cell mass and the trophectoderm. This gives us more insight into developmental potential.
If only one embryo is available, transferring earlier may be appropriate, as it allows the embryo to develop in the uterine environment. When multiple embryos are available, extended culture allows better selection of the embryo with the highest implantation potential. These decisions are always discussed with the patient and tailored to their situation.
Dr Renata Huttelova, PhD, Head of Embryology Lab, IVF CUBE, Czechia: I agree. Today, the general goal is blastocyst transfer on day 5, although there are advantages and disadvantages. The uterus is the natural environment for a blastocyst, which supports this approach. However, laboratory conditions and individual embryo sensitivity must also be considered.
Some embryos may benefit from earlier transfer rather than extended culture. Each laboratory has general policies, but there should always be flexibility to individualise treatment. The aim is to determine what is best for each patient.
David O’Rourke, IVF Lab Director, Freya Fertility, Denmark: This is a very common and important question. When discussing this with patients, we often start with the end goal—achieving a pregnancy—and then work backwards.
Maternal age is one of the biggest factors. There has been extensive analysis of how many oocytes and embryos are typically needed to achieve a pregnancy, particularly when considering aneuploidy rates.
If we consider a hypothetical cycle for a 40-year-old patient, and assume that 10 oocytes are collected, we would expect approximately seven of those to fertilise. From those seven fertilised oocytes, around 40% might develop into blastocysts of sufficient quality to either transfer or freeze. That would result in approximately three blastocysts.
At the age of 40, the chance of a blastocyst being euploid—genetically normal and chromosomally balanced—is estimated to be between 10% and 20%. This percentage decreases significantly with age, dropping from around 70% at age 30 to closer to 10–20% by age 40.
Based on large datasets, this would suggest that roughly 10 blastocysts may be needed to achieve one euploid embryo. However, this is an estimate based on averages. Some patients may achieve a euploid embryo in their first cycle, while for others it may take several cycles.
It is therefore very difficult to predict exactly how many cycles an individual patient will need. These figures are meant to provide a general framework rather than a definitive answer.
Dr Renata Huttelova, PhD, Head of Embryology Lab, IVF CUBE, Czechia: I agree with David’s explanation. I would add that I am perhaps more cautious when estimating euploid rates in patients over 40. In my experience, a 20% euploid rate at this age would be considered very good. In many cases, the rate is closer to 10% or even lower.
David O’Rourke, IVF Lab Director, Freya Fertility, Denmark: We benchmark ourselves against established guidelines but always aim to exceed them. Our minimum benchmark fertilisation rate is 70%, while our actual average fertilisation rate is closer to 80–85%.
There are several reasons for this. We perform extensive semen analysis before treatment and try to gather as much information as possible about patients in advance. If fertilisation potential is unclear, especially in a first cycle, we often perform split insemination. This helps minimise the risk of failed fertilisation.
Split insemination means dividing oocytes between conventional IVF and ICSI. This approach helps mitigate risk and maximise outcomes.
In terms of embryo development, there are two main philosophies. We have largely moved towards a single-step culture system, where embryos are left undisturbed. We use time-lapse incubators, which take images every 10–15 minutes and allow us to monitor development continuously without removing embryos from optimal conditions. This also supports embryo selection by identifying both positive and negative developmental events.
With single-step culture and time-lapse incubation, we generally see around a 40% blastocyst utilisation rate, meaning embryos suitable for transfer or freezing. Our overall blastocyst formation rate can be closer to 50%, but around 10% of those embryos may not be suitable for freezing due to poor quality. Very low-quality embryos do not survive vitrification well, and some may lack essential cell types, such as an inner cell mass, which is required for continued development and live birth.
When we look beyond blastocyst formation to implantation and live birth rates, these are strongly age-dependent. Pregnancy and live birth rates are highest in patients aged 35 and under, who have the lowest incidence of aneuploidy and approximately a 50% live birth rate. This declines significantly as patients approach 40 and beyond.
Dr Renata Huttelova, PhD, Head of Embryology Lab, IVF CUBE, Czechia: I would like to emphasise some general principles. First, biology is never 100% predictable. If we retrieve 10 eggs, we cannot expect all of them to be mature, fertilised, or develop into embryos. This is something that must be clearly explained to patients, as many expect 10 eggs to result in 10 embryos.
A good laboratory monitors KPIs, but these figures represent averages across patient populations. Individual patients can differ significantly. Even if a clinic reports an 85% fertilisation rate overall, some patients may have only 20% fertilisation, while others may have 100%.
These variations can strongly affect how patients feel during treatment. While statistics help guide expectations, everything must be tailored to the individual. There are many factors that influence outcomes, and it is extremely difficult to predict results with certainty.
Ultimately, our goal is to achieve a pregnancy and a live birth. We use statistics to guide decisions, but they are only estimates. We do our best work, but we can never make guarantees.
David O’Rourke, IVF Lab Director, Freya Fertility, Denmark: It is easy to discuss high-level numbers, but they reflect hundreds of cycles. Each individual cycle is different, and biology can be both surprising and cruel. The statistics do not always represent the human experience behind them.
David O’Rourke, IVF Lab Director, Freya Fertility, Denmark: Practices vary widely between countries and clinics. Many clinics maintain a balance between IVF and ICSI rather than defaulting to ICSI.
If ICSI is used in 100% of cases, it does provide more information on day zero, such as oocyte maturity and a tactile sense of egg quality during injection. However, ICSI removes an element of natural sperm selection.
Whether this affects blastocyst quality is unclear. I do not see consistent evidence that one method is superior in terms of attrition. There is no clear answer, as many factors are involved.
Dr Renata Huttelova, PhD, Head of Embryology Lab, IVF CUBE, Czechia: Long-term studies show no difference in health outcomes between children born after IVF or ICSI, confirming that ICSI is a safe technique.
Clinic policies may also influence ICSI use, particularly in donor programs where maximising fertilisation is critical. Time-lapse incubation often requires removal of cumulus cells, which necessitates ICSI and allows early assessment of egg quality.
Some patients benefit from IVF, others from ICSI, and decisions should always be individualised. There are cases where egg quality is poor, and fertilisation requires additional assistance.
David O’Rourke, IVF Lab Director, Freya Fertility, Denmark: Donor programs rely on extensive raw data collected over time. By analysing how many mature oocytes are allocated, fertilised, and develop into blastocysts, programs can estimate outcomes with reasonable confidence.
Donors are carefully selected, thoroughly tested, and often have proven fertility, giving them the highest prognosis. While biology is never 100% predictable, these factors allow clinics to confidently guarantee a blastocyst in many cases.
Strong vitrification protocols and laboratory expertise are essential, particularly because freezing oocytes is technically more challenging than freezing blastocysts.
Dr Renata Huttelova, PhD, Head of Embryology Lab, IVF CUBE, Czechia: It is very difficult to change egg quality, as eggs are formed over a woman’s reproductive lifetime. A healthy lifestyle, avoiding smoking and alcohol, and using antioxidants may help, but they will not result in dramatic improvements.
Each stimulation cycle recruits a different cohort of follicles. According to cohort theory, eggs from the same cycle tend to have similar quality. Outcomes can vary significantly between cycles, even with the same patient and protocol. One cycle may produce higher-quality blastocysts than another, and success may depend on which cohort is used.
There are many unknown factors, and unfortunately, there is little we can do to fully control these outcomes.
David O’Rourke, IVF Lab Director, Freya Fertility, Denmark: Lifestyle and dietary factors may help protect the ovarian environment, but their impact is limited. Antioxidants can reduce oxidative stress, but this may not be the main issue affecting egg quality.
Even when patients do everything right, outcomes can still vary. It is important to understand that some factors are beyond personal control.
