Blastocyst Transfer: Day 3 vs Day 5

One of the key decisions in IVF is when to transfer your embryo. Some transfers happen on day 3, when the embryo has around 6–8 cells, while others take place on day 5, when it has reached the blastocyst stage - a more advanced form with hundreds of cells and a fluid filled cavity.

The choice can influence success rates, embryo selection, and even how many embryos you have available for freezing.

What Happens Between Day 3 and Day 5?

After fertilisation, the embryo divides and grows:

• Day 3 (Cleavage Stage): About 6–8 cells, each still relatively identical. The embryo hasn’t yet started to differentiate into distinct cell types.

• Day 5 (Blastocyst Stage): Around 200+ cells, now forming two types: the inner cell mass (future baby) and the trophectoderm (future placenta).

The jump from day 3 to day 5 is significant - many embryos stop developing during this period, which is why some clinics prefer to wait and see which ones reach blastocyst.

Day 3 Transfer: Pros & Cons

Pros:

• More embryos available for transfer - some may not survive to day 5 in the lab but could still thrive in the uterus.

• Useful if you have only a few embryos, as it avoids losing them in extended culture.

• Transfer happens earlier, which some patients prefer emotionally.

Cons:

• Less information about embryo quality at this stage.

• Higher chance of transferring an embryo that won’t implant.

Day 5 (Blastocyst) Transfer: Pros & Cons

Pros:

• Better embryo selection - only the strongest embryos make it to blastocyst.

• Higher implantation rates in many cases.

• Allows for single embryo transfer with good success, reducing multiple pregnancy risk.

• Easier to time genetic testing results.

Cons:

• Some embryos won’t survive to day 5, which can be devastating if numbers are already low.

• Requires excellent lab conditions and expertise.

How Timing Affects Success Rates

Research suggests blastocyst transfers often have slightly higher success rates, especially in younger patients or those with more embryos. However, for patients with fewer embryos or certain medical conditions, a day 3 transfer may be the more cautious option.

Success depends not just on timing, but also on:

• Embryo quality

• Uterine lining readiness

• Your age and fertility history

How Clinics Decide

Your clinic will consider:

• Number of embryos available

• Quality seen in the lab

• Past IVF history

• Whether genetic testing is planned

• Your personal preferences

Bottom Line

There’s no one size fits all answer. Day 5 transfers may offer better embryo selection and higher implantation rates, but day 3 transfers can be a smart strategy when embryo numbers are limited. The right choice balances embryo survival, lab expertise, and your treatment goals.