The easy answer to this question: the best embryos are chosen!
There are a number of factors that determine which embryos are deemed best. On any given day of culture, the embryos are expected to have a specific appearance:
- On Days 2 and 3, they should be two, four, or eight cells
- They should be a morula on Day 4
- They should be at blastocyst stage by Day 5 or Day 6
With an embryo transfer, it’s important to choose only embryos that look viable given the day of culture. For example, we don’t want to transfer an embryo on Day 5 that looks like a Day 3 embryo, which has surely stopped growing and won’t implant and grow.
An additional factor is when to transfer the embryos. Embryos generally look similar on Day 3 and become more diverse in appearance by Day 5. By waiting until Day 5, the embryos with a better chance of success are more evident and can be selected. There are two issues to consider:
- Whether to transfer the embryos on Day 3 without knowing if they are going to make it to a blastocyst.
- Whether to wait until Day 5 to see if they make it to the blastocyst stage, risking the chance of not having a transfer at all.
If the lab has a good culture (embryo growing) system, the outcome will probably be embryos (and not blastocysts) if you transfer on either day. The embryos will, hopefully, continue to develop, but may not in either case.
Keep in mind that the difference is the disappointment of not having a transfer versus knowing sooner and stopping the medication and restrictions. It’s not always an easy decision to make, and it is suggested that you follow the guidance of your doctor and embryologist.