Do all embryos become a blastocyst?

Not all fertilised eggs reach the blastocyst stage. Only a proportion of normally fertilised embryos fully develop to Day 5. Development can stop at any stage, as embryo growth depends on egg quality, sperm factors, and the natural variability of early cell division.

Does a blastocyst transfer improve success rates?

A blastocyst transfer may improve success rates because the embryo has already progressed to a strong developmental stage and is more synchronised with the uterine environment. This can increase the chance of implantation, particularly when high‑quality blastocysts are available and carefully selected for transfer.

What affects embryo development?

Embryo development is influenced by egg quality and age, sperm health, and the laboratory environment supporting early growth. As eggs age, blastulation rates typically decline, affecting the number of embryos that reach the blastocyst stage. Optimal culture conditions and genetics also play key roles.

Does blastocyst culture have any potential risks?

The risk of blastocyst culture is that embryos may not develop to the blastocyst stage, and the IVF cycle is cancelled with no embryo transfer taking place. There are many factors influencing this risk, including, but not limited to, patient egg age, egg quality, number of eggs collected, body mass index, sperm quality, poor embryo development, and genetic factors.

Scientific Techniques

The Stages of Embryo Development

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Blastocyst embryo transfer for IVF treatment

A blastocyst embryo transfer involves placing a Day 5 embryo, known as a blastocyst, into the uterus at the stage it naturally reaches before implantation.

In IVF, allowing embryos to develop to the blastocyst stage helps embryologists observe their growth and select those with the highest implantation potential. This targeted approach supports higher IVF success rates because blastocysts contain the early structures needed for attachment and development.

By transferring a carefully selected blastocyst, patients benefit from a more physiologically timed embryo transfer and a reduced need for transferring multiple embryos. This helps optimise implantation outcomes and supports a safer, single‑embryo transfer strategy.

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Cell structure of a blastocyst

A blastocyst contains two key cell groups: the trophectoderm (TE) and the inner cell mass (ICM).

The TE cells form the outer layer of the blastocyst and play a crucial role in implantation, eventually developing into the placenta.

The ICM is a compact cluster of cells inside the blastocyst that will develop into the foetus. These cells develop into all the body’s major cell types, and they are also the source of embryonic stem cells used in research.

Understanding the relationship between the TE and ICM helps embryologists assess embryo potential at the blastocyst stage.

How do we know how an embryo is developing?

Embryologists monitor how an embryo is developing by observing its growth and changes in the laboratory. At the blastocyst stage, they can see enough detail to assess its overall development. Using tools such as the Gardner grading system, embryologists review the appearance and organisation of the blastocyst to determine which embryos are developing well and are most suitable for transfer or freezing.

If you are interested in the Gardner Embryo Grading System, our Victorian Scientific Director, Jayne Mullen, has prepared a blog on this

If you would like to learn more about the stages of embryo development, read here: The Stages of Embryo Development| City Fertility

Blastocyst Stage Extended Culture (Day 5 Embryo Transfer)

In a typical IVF cycle, not all normally fertilised eggs will reach the blastocyst stage of development. Generally, a higher number of fertilised eggs leads to a higher overall blastulation rate. Blastulation rate refers to the percentage of embryos that form blastocysts.

Egg number and the age of the eggs can significantly affect the blastulation rate and, consequently, the potential for successful outcomes.

What happens to a blastocyst after embryo transfer?

After a blastocyst embryo transfer is completed, it takes about nine days before a pregnancy can be detected. During the days after a transfer, the following happens to the embryo:

Day 1: The blastocyst begins hatching out of its shell.
Day 2: Hatching continues, and the embryo adheres to and starts attaching to the endometrium.
Day 3: The embryo embeds more deeply as implantation begins.
Day 4: Implantation progresses further into the uterine lining.
Day 5: Implantation is complete. Cells that eventually become the placenta and foetus have begun to develop.
Day 6: The pregnancy hormone, hCG, starts entering the bloodstream, signalling pregnancy.
Days 7–8: Early fetal development continues, and hCG production increases.
Day 10: hCG levels are typically high enough to confirm pregnancy via a blood test.

Frequently Asked Questions

Do all embryos become a blastocyst?
Not all fertilised eggs reach the blastocyst stage. Only a proportion of normally fertilised embryos fully develop to Day 5. Development can stop at any stage, as embryo growth depends on egg quality, sperm factors, and the natural variability of early cell division.
Does a blastocyst transfer improve success rates?
A blastocyst transfer may improve success rates because the embryo has already progressed to a strong developmental stage and is more synchronised with the uterine environment. This can increase the chance of implantation, particularly when high‑quality blastocysts are available and carefully selected for transfer.
What affects embryo development?
Embryo development is influenced by egg quality and age, sperm health, and the laboratory environment supporting early growth. As eggs age, blastulation rates typically decline, affecting the number of embryos that reach the blastocyst stage. Optimal culture conditions and genetics also play key roles.
Does blastocyst culture have any potential risks?
The risk of blastocyst culture is that embryos may not develop to the blastocyst stage, and the IVF cycle is cancelled with no embryo transfer taking place. There are many factors influencing this risk, including, but not limited to, patient egg age, egg quality, number of eggs collected, body mass index, sperm quality, poor embryo development, and genetic factors.