Written by Rhiannon Davis. Reviewed by Dr. Phoebe Howells.

When it comes to IVF, the embryo transfer stage is a critical milestone. By this point, you've likely been through weeks of hormone injections, an egg retrieval, and now face the decision of which embryo to transfer. But is a day 3 embryo better, or should you wait for a day 5 blastocyst? And why is it day 3 and day 5?

Understanding Embryo Development

Before an embryo can be transferred to the uterus, your embryologist will assess which one has the best chance of leading to a viable pregnancy. The timing of this transfer, whether on day 3 or day 5, plays a key role in success rates.

Why not day 4?

Before we get into the details of day 3 vs day 5, let’s go into the mysterious missing Day 4… In IVF, Day 4 is a bit of a no-man's land in embryo development. Day 4 is a transitional phase, where the embryo is "compacting", cells are merging together and starting to form the early structure of a blastocyst. At this stage, the embryo doesn't give us much to work with visually. It can't be graded or assessed properly under a microscope like Day 3 or Day 5 embryos.

Day 3 Embryo (Cleavage Stage)

A day 3 embryo is in the cleavage stage, where it has divided from a single cell into a cluster, typically around 6-8 cells. These are called blastomeres, and although the number increases, the embryo stays roughly the same size overall. Imagine slicing a pizza into smaller and smaller pieces without changing the size of the pizza. Embryologists grade these embryos based on the number and appearance of the cells. An ideal day 3 embryo will have evenly sized cells with minimal fragmentation (small bits of cellular debris).

The argument for day 3 transfers is that some embryos may thrive better in the uterine environment than in the lab, where conditions can’t fully mimic the body. However, day 3 embryos are less developed, so it's harder to predict which ones are genetically normal and capable of implanting. Day 3 transfers are often considered for people who have a limited number of embryos. Rather than risk them arresting in the lab, transferring earlier gives them a chance to continue development in the uterus.

Day 5 Embryo (Blastocyst Stage)

By day 5, an embryo reaches the blastocyst stage. At this point, the embryo has differentiated into two types of cells: the inner cell mass (which will become the fetus) and the trophectoderm (which will form the placenta). Day 5 embryos are graded based on how well these structures are developing.

Not all embryos will make it to the blastocyst stage, only the strongest tend to survive this far. This means that reaching Day 5 is the body’s way of supporting the strongest embryo to thrive.

The benefit of waiting until day 5 is that embryologists can better identify which embryos are most likely to implant. Day 5 embryos can also undergo preimplantation genetic testing (PGT), which tends to be more accurate at this stage due to the higher number of cells available for biopsy. This screens for atypical chromosomes, increasing the chances of a successful pregnancy. Plus, the timing of a day 5 transfer more closely mimics natural implantation, which typically happens around days 5-7.

In short:

Day 3 = early potential
Day 4 = awkward teenage phase
Day 5 = fully developed blastocyst, ready to implant

So, Which is Better, Day 3 or Day 5?

It’s not a simple case of one being better than the other – it depends on your individual circumstances.

• Day 3 transfers may be recommended for those with fewer embryos, as it gives them a chance to implant in the uterus rather than being lost in the lab.

• Day 5 transfers generally have higher implantation rates and allow for genetic testing, but not all embryos make it to this stage.

In some cases, your clinic may recommend a "sequential transfer", where embryos are transferred on both Day 3 and Day 5, but this is less common and usually part of a personalised protocol.

What Does the Research Say?

While some studies show higher success rates with day 5 transfers, others suggest that transferring more embryos on day 3 can achieve similar outcomes. The latest evidence indicates that there’s no one-size-fits-all approach. Your clinic will consider your age, egg quality, and any previous IVF outcomes when advising you. The HFEA offer guidance on how the decision is made in the UK.

Some clinics also freeze embryos on both Day 3 and Day 5 to preserve all options and tailor future transfers to the patient’s response.

The OVUM Takeaway

We believe in empowering you with science-backed, evidence-led knowledge so you can make informed choices. Whether you opt for a day 3 or day 5 transfer, what matters most is working with a clinic that understands your unique biology and supports you every step of the way.