40w LaterGuide

During IVF, you may hear about embryos being transferred on Day 3 or Day 5. What is the difference, and how is the timing of embryo transfer decided?

What are Day 3 and Day 5 embryos?

After eggs are retrieved and fertilized with sperm, the resulting embryos continue to develop in the laboratory.

About three days after fertilization, an embryo usually reaches the cleavage stage, when it consists of several cells. Transferring an embryo at this stage is commonly called a Day 3 embryo transfer.

With continued culture, some embryos develop into blastocysts around Day 5 or 6. Transferring an embryo at this stage is commonly called a Day 5 or blastocyst transfer.

Not all embryos develop at the same rate, so some may reach the blastocyst stage on Day 6 rather than Day 5.

Is a Day 5 embryo better than a Day 3 embryo?

Not always. Extending culture to the blastocyst stage allows the laboratory to observe embryo development for longer and may help select an embryo for transfer.

In people with a good prognosis, blastocyst transfer may offer a higher chance of pregnancy and live birth per transfer than cleavage-stage transfer.

However, not every cleavage-stage embryo will develop into a blastocyst. Extending culture may therefore result in having no embryo available for transfer.

Rather than thinking that culturing an embryo until Day 5 makes it “better,” it is more accurate to say that reaching the blastocyst stage provides additional information that can help with embryo selection.

Does embryo grading matter?

Embryo development and morphology are among the factors used to select embryos for transfer. Day 3 embryos are generally assessed by characteristics such as cell number, symmetry and fragmentation, while blastocysts are evaluated according to their stage of development and the appearance of the inner cell mass and trophectoderm.

However, embryo grade alone cannot accurately predict whether implantation, pregnancy or live birth will occur.

Is transferring more embryos better?

No. Transferring more embryos does not increase the chance of pregnancy in direct proportion to the number transferred, but it does increase the risk of multiple pregnancy.

For this reason, single embryo transfer (SET) is preferred in many situations to reduce the risk of twins or higher-order multiple pregnancies. The number of embryos transferred is determined according to factors such as age, embryo development and quality, previous IVF outcomes, genetic testing and local guidelines.

So, should you choose Day 3 or Day 5?

There is no single best choice for everyone. The decision may depend on the number and development of available embryos, age, previous treatment outcomes, laboratory experience, whether the transfer is fresh or frozen, and whether preimplantation genetic testing is planned.

If several embryos are developing well, extended culture to the blastocyst stage may help with embryo selection. If only a small number of embryos are available, the possibility of having no embryo available after extended culture may also need to be considered.

Ultimately, neither Day 3 nor Day 5 is always better. The most appropriate timing should be chosen according to the embryos available and the individual treatment situation.