Does BMI Influence IVF Success? What We Found After Analysing 10,924 Embryo Transfer Cycles
Does a higher BMI really mean a lower chance of IVF success?
It’s a question many patients worry about before they even walk through the doors of a fertility clinic. Weight and BMI have long been part of the conversation around fertility, and women with a higher BMI may assume that the number on the scale automatically means a lower chance of success.
But when researchers at No.1 Fertility analysed 10,924 embryo transfer cycles across seven years, the data told a more complex story.
BMI itself was not predictive of clinical pregnancy.
The research, titled Does BMI Influence IVF Success? Insights From A Large Retrospective Cohort Study, was conducted by Shirley Zhang, Dr Kerri Johnson, Dr Lynn Burmeister and Dr Kelli Sorbyfrom No.1 Fertility in Melbourne.
The findings add an important Australian perspective to a question that remains debated in reproductive medicine — and challenge us to consider whether we sometimes place too much emphasis on a single number when fertility is far more complex.
Looking at seven years of IVF data
The No.1 Fertility research team conducted a retrospective analysis of all fresh and frozen embryo transfer cycles included in the study between January 2018 and December 2024.
Patients were grouped according to Australian Government BMI classifications, from underweight and healthy weight through to overweight and obesity classes I, II and III.
Of the 10,924 transfer cycles analysed, 60.7% were in the healthy-weight group, 21.6% in the overweight group, 9.1% in obesity class I, 3.4% in obesity class II, 1.1% in obesity class III and 4% in the underweight group.
The researchers compared a range of important IVF measures across these groups, including patient age, the number of oocytes collected, blastocysts vitrified per egg collection, clinical pregnancy rates, miscarriage rates and live birth rates. A separate subgroup analysis also examined patients with and without polycystic ovary syndrome (PCOS).
This allowed the team to look beyond BMI alone and explore some of the other factors that may influence fertility treatment outcomes.
So, what did we find?
One of the most significant findings came from the team’s logistic regression analysis.
BMI was not found to be predictive of clinical pregnancy. Age and PCOS, however, were predictive factors in the analysis.
The data also produced some interesting results when individual BMI groups were compared. The highest clinical pregnancy rate was observed in the obesity class II group, at 43.1%, compared with 37.5% in the healthy-weight group, despite the two groups having a similar average patient age.
Oocyte yield and the number of blastocysts vitrified per egg collection were also broadly comparable across BMI groups.
The study did identify differences in miscarriage rates. The obesity class II group recorded the highest miscarriage rate, which the researchers found was statistically significant compared with the underweight, healthy-weight and overweight groups. This is an important reminder that IVF outcomes are complex and that a clinical pregnancy is only one part of the journey towards a live birth.
The subgroup analysis revealed another interesting pattern. Across all BMI groups, patients with PCOS were younger and had higher pregnancy rates than patients without PCOS.
When these factors were considered together, the research suggested something important: BMI alone may not tell us whether someone is likely to have a good or poor IVF outcome.
Why this research matters
BMI is a simple number to calculate. Fertility is far more complex.
The likelihood of IVF success can be influenced by many interconnected factors, including age, ovarian reserve, egg and sperm quality, embryo development, underlying reproductive and medical conditions, and a person’s individual fertility history.
Our findings suggest that looking at BMI in isolation may not provide the full picture.
This matters because patients with a higher BMI can sometimes feel that their weight defines their reproductive potential before their individual circumstances have been fully considered.
The conclusion of our research poster is deliberately simple:
High BMI may not be indicative of a low IVF outcome.
That does not mean BMI, weight or metabolic health are unimportant. They can have implications for a person’s overall health, pregnancy risks and the safety and management of some medical procedures. Every patient should be assessed individually by their treating team.
What this research does suggest is that BMI should be considered as one part of a much larger fertility picture, rather than being viewed in isolation as a predictor of IVF success.
Researching the questions that matter to our patients
At No.1 Fertility, we believe research should help us better understand the questions our patients face in the real world.
This study was developed by members of our own team using our clinical data to investigate a question that continues to generate discussion internationally. By analysing 10,924 embryo transfer cycles across seven years, our researchers have contributed valuable Australian data to the conversation about BMI and IVF outcomes.
Research like this is also part of a broader philosophy at No.1 Fertility. We don’t believe fertility care should be reduced to a single number, whether that number is age, BMI or any other individual measure. Good fertility medicine means looking at the person in front of us, understanding their individual circumstances and using the available evidence to guide their care.
For us, that may be the most important message to come from this research.
A number on a scale cannot tell the whole story of someone’s fertility. And based on this large retrospective analysis, BMI alone did not predict clinical pregnancy.
About the research
Does BMI Influence IVF Success? Insights From A Large Retrospective Cohort Study
Authors: Shirley Zhang, Dr Kerri Johnson, Dr Lynn Burmeister and Dr Kelli Sorby
Institution: No.1 Fertility, Melbourne, Australia
