Could some "abnormal" embryos still be safe to transfer?
Key takeaway
- Some embryos with a single pronucleus (1PN) are widely discarded as abnormal, yet a proportion are genetically normal.
- This large Genea led study analysed over 13,000 such embryos across more than 10,700 IVF cycles.
- 1PN embryos from standard IVF had a 96.9% rate of normal, two parent genetics, far higher than those from ICSI.
- Where genetics were confirmed normal, live birth and baby outcomes matched standard embryos.
- No molar pregnancies occurred across 257 transfers of these embryos.
What is a 1PN embryo, and why are they usually discarded?
Around 16 to 18 hours after eggs are fertilised in the laboratory, embryologists check for the presence of two pronuclei, one from the egg and one from the sperm. This is the visual sign of normal fertilisation. Occasionally, only a single pronucleus is seen instead of two, known as a 1PN or monopronuclear embryo.
Historically, and under most current international guidelines, embryos identified as 1PN are not used for transfer. They are considered atypical, and the concern has been that they may not contain the normal two parent (biparental) set of chromosomes needed for a healthy pregnancy. As a result, many clinics discard these embryos automatically, without further testing.
However, a growing body of research, including Genea's own earlier work, has shown that a meaningful proportion of 1PN embryos are actually genetically normal, and can go on to result in healthy pregnancies and live births when identified and used appropriately.
The question this study set out to answer
This study, published in Human Reproduction, set out to build the largest and most detailed picture yet of what happens when 1PN embryos are properly genetically tested rather than automatically discarded, and whether outcomes differ depending on how the embryo was created, through standard IVF insemination or ICSI (intracytoplasmic sperm injection, where a single sperm is injected directly into the egg).
The research team, led by Genea's Dr Nicole McPherson and Dr Dean Morbeck, analysed 13,203 1PN embryos from 10,730 IVF and ICSI cycles performed at Genea clinics across Australia between 2010 and 2024, making this the largest retrospective study of its kind to date.
What the research found, in more detail
Why this matters for future IVF guidelines
Most existing international guidelines recommend against transferring any 1PN embryo, largely based on evidence from ICSI cycles, where the risk of abnormal genetics is higher. This study is one of the first to separate out IVF derived 1PN embryos specifically, and its findings suggest they may carry a meaningfully lower risk profile than has previously been assumed.
The authors are careful to frame this as an important step toward future guideline development, not an immediate call for universal change. Rather, they propose that IVF derived 1PN embryos meeting specific developmental and morphological criteria could be considered individually, alongside genetic testing, when discussing options with patients, particularly in cycles where no standard embryo is available.
At Genea, IVF derived 1PN embryos meeting appropriate criteria may be considered for transfer without mandatory genetic testing, particularly for patients who have not otherwise opted for PGT. ICSI derived 1PN embryos, given their higher rate of abnormal genetics, still require genetic confirmation of normal, two parent inheritance before being considered for transfer. This research adds significant, real world evidence to support and refine that risk stratified approach over time, as international guidelines continue to evolve.
What this means for you
If you are undergoing IVF, this research offers a genuinely hopeful message for a specific, limited situation, if a 1PN embryo forms during your cycle, it does not automatically mean that embryo cannot contribute to your family. With proper genetic testing to confirm normal, two parent chromosomes, it may still be a safe and valuable option to consider, particularly if few or no other embryos are available.
This does not change how the vast majority of embryos are assessed and used in your IVF cycle. Your embryology team will always discuss your specific embryos and options with you individually, based on your own circumstances and test results.
FAQs
About the authors
Dr Anthony Marren
Medical Director Sydney CBD and CREI Fertility Specialist B. Med (Hons.), M. Med (R. H. H. G.), FRANZCOG, CREI
Sydney CREI Specialist Dr Anthony Marren is the Medical Director at Genea Sydney CBD. Leading expert in IVF, fertility & recurrent pregnancy loss treatment.
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Dr. Nicole McPherson
Director of Research and Diagnostic Laboratories
Dr Nicole McPherson is Genea Director of Research, Senior Lecturer and Hospital Research Fellow who leads the Sperm and Embryo Biology Group at Adelaide Uni.
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Dr. Dean Morbeck
Genea Chief Scientific Officer
Dr. Dean Morbeck, PhD, HCLD, is Chief Scientific Officer for Genea Fertility and adjunct Associate Professor at Monash University.
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Diana Bui
Diana Bui - Genea Director of Clinical Embryology
Diana Bui, Director of Clinical Embryology at Genea, has been part of the Genea Embryology team for over 13 years and an Embryologist for over 21 years.
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Dr. Mark Bowman
Previous Medical Director at Genea
Associate Professor Dr Mark Bowman is a Sydney based gynaecologist and fertility specialist, known for his work in reproductive endocrinology and IVF.
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Rebecca Dickson
Senior Genetic Counsellor
Genetic counsellor with over 16 years experience across prenatal, paediatric, adult and cancer genetics, now genetic counselling manager at Genea Fertility.
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References
- McPherson, N. O., Ussher, C., Holden, S., Hesketh, N., Murray, A., Lee, A., Bui, D., Traversa, M. V., Grkovic, S., Dickson, R., Bowman, M., Marren, A., & Morbeck, D. E. (2026). A large retrospective study on 1PN embryo transfer supports the need for updated harmonized best practice guidelines. Human Reproduction, 00(00), 1-17. https://doi.org/10.1093/humrep/deag098
Want to understand how your embryos are assessed?
Our Fertility Concierge team can connect you with a specialist or embryologist to explain how embryo selection and genetic testing works at Genea.