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OverviewDay 5/6 TransferSingle Embryo TransferResearchFAQs
As an early adopter of Day 5/6 embryo transfer and single embryo transfer, Genea helped drive these advances across the Australian fertility industry, where they are now standard practice. 

Here we explain what each approach involves, the biology behind how they work, the clinical evidence supporting them, and what they mean in practice for patients going through IVF at Genea. 

What is a Day 5/6 embryo transfer?

After fertilisation, an embryo goes through rapid cell division over the following days. In earlier IVF practice, embryos were typically transferred on Day 2 or Day 3, when they had reached a 4 to 8 cell stage known as the cleavage stage. Genea moved away from this approach and now routinely transfers embryos on Day 5/6, when they have developed into blastocysts.

A blastocyst is a more complex, more developed structure with roughly 200 cells arranged into two distinct cell types: the inner cell mass, which will become the baby, and the trophectoderm, which will become the placenta. Not all embryos that fertilise successfully will reach this stage, and it’s a meaningful milestone that demonstrates the embryo’s developmental capacity.

Key differences between Day 2/3 vs Day 5/6 transfers

Day 2/3 cleavage stageDay 5/6 blastocyst stage
6-8 cellsAround 200 cells, two distinct cell types
Less information about embryo viabilityDemonstrated developmental capacity
Potentially more embryos available to transferPotentially fewer embryos available to transfer, but better selected
Lower individual implantation rateHigher individual implantation rate
Often involves transferring more than one embryo, which increases the rate of multiple pregnanciesSupports single embryo transfer

Our technology and expertise allow us to maintain high success rates while transferring a single, carefully selected embryo. This approach protects the health of our patients and their babies without compromising the outcomes they come to us for - Dean Morbeck, Chief Scientific Officer at Genea Fertility Sydney

How does Day 5/6 transfer impact IVF outcomes?

There are three key reasons why waiting for the blastocyst stage before transfer is clinically significant.

1. Natural selection

Embryos that appear healthy on Day 3 may have chromosomal or developmental abnormalities that prevent them from reaching the blastocyst stage. Extended culture acts as a natural filter, as embryos that reach Day 5/6 have already demonstrated a key developmental capacity and are more likely to implant successfully.

Does extended culture risk losing embryos?

Some embryos that reach Day 3 will not develop into a blastocyst in the laboratory. For patients with fewer embryos, this is an understandable concern, and your Genea specialist will discuss this with you based on your individual situation.

For most patients, however, the embryos that don't reach Day 5/6 in culture would also be unlikely to implant successfully if transferred earlier. Extended culture reveals, rather than causes, that outcome.

Genea's ongoing investment in laboratory technology, including the Geri® time-lapse incubator developed by Genea Biomedx, is specifically designed to optimise conditions for blastocyst development and reduce risks associated with extended culture.


What is single embryo transfer?

Single embryo transfer (SET) means transferring one embryo per cycle rather than two or more. Genea was an early adopter of this approach in Australia, and it remains central to our IVF approach today.

For most patients, transferring two embryos does not meaningfully increase the chance of having a baby, but it does significantly increase the chance of a twin pregnancy. And while a twin pregnancy can feel like a welcome outcome, it carries substantially higher health risks for both mother and babies than a single pregnancy. 

Does single embryo transfer lower my chances of success?

Elective single embryo transfer combined with blastocyst-stage selection does not lower the cumulative chance of a live birth. Here's why.

If a first transfer is unsuccessful, any remaining frozen embryos can be thawed and transferred in a subsequent cycle. The cumulative live birth rate over two sequential single embryo transfers is comparable to one double embryo transfer, but with a much lower risk of twins.1

This is one of the reasons Day 5/6 transfer and single embryo transfer are part of the IVF process at Genea. Better embryo selection at the blastocyst stage gives us the confidence to transfer one embryo, because we've chosen the one most likely to implant.

FAQs

Blastocyst transfer and single embryo transfer as a combined approach

These two practices reinforce each other, as extended blastocyst culture provides the embryology team with information needed to make a well-grounded single-embryo selection. That confidence in selection is part of what makes it clinically appropriate and safe to transfer just one embryo.

When preimplantation genetic testing (PGT) is used alongside blastocyst biopsy at Day 5/6, this may further increase confidence in embryo selection, particularly for patients at higher risk of chromosomal abnormalities or those who have experienced recurrent implantation failure or pregnancy loss.

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References

  1. McArthur, S. J., Leigh, D., Marshall, J. T., de Boer, K. A., & Jansen, R. P. S. (2005). Pregnancies and live births after trophectoderm biopsy and preimplantation genetic testing of human blastocysts. Fertility and Sterility, 84(6), 1628–1636. https://doi.org/10.1016/j.fertnstert.2005.05.063.
  2. Genea. (2023). Frozen Embryo Transfer Success Rates: Clinical Pregnancy and Live Birth Rates by Age Group, January 1 - December 31, 2022. Internal clinical data report. Sydney: Genea Fertility Clinics. Note: Data represents autologous cycles only, includes all PGT-tested embryos, excludes RPA, Fertility SA and Fertility First clinics

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