Day 5 (6 or 7)

Embryos that continue to divide until day 5 undergo several more cell divisions.

In principle, all blastomeres double each time, creating a series: 8, 16, 32, 64, 128,….

On day 4, compaction occurs: the cells form one compact unit with no more visible individual cells. On day 5, the embryo shows a cavity. We have now arrived at the stage of the blastocyst. We can no longer count the cells individually.
Still we can distinguish two cell compartments: an inner cell mass, which will form the future foetus, and an outer layer of cells called the trophectoderm, which will form the future placenta.

Embryos in this stage are assigned different scores depending on their development stage:

  • early blastocysts with a smaller cavity and no inner cell mass development yet, versus
  • full blastocysts with a larger cavity and inner cell mass development.

The number of cells in both cell compartments and the way they are grouped will also contribute to the embryo’s score.

Five common questions about embryology

Why has my embryo transfer been postponed from day 3 to day 5?

If the embryo transfer is postponed from day 3 to day 5, this means that at least four good quality embryos are present. In order to make a better selection, we wait until day 5 to make a final choice of the embryo or embryos showing the best development pattern.

Why am I not getting an embryo transfer?

Only embryos up to grade 3 are eligible for transfer to the uterus. If we do not have any embryos of this quality, we have to cancel your embryo transfer. The embryos may not have divided sufficiently, may have stopped developing or showed too much fragmentation. One medical reason may be that you are experiencing hyperstimulation. Going ahead with an embryo transfer in the current cycle would only make this problem worse, as the embryo will also start to produce hormones during its further development in your body. Your physician will adjust your stimulation plan to avoid hyperstimulation during a possible subsequent attempt. If sufficient good-quality embryos are available, we will freeze them for possible transfer later. In a cycle with thawed embryos, the embryo transfer is not preceded by a (major) stimulation stage, so no hyperstimulation can occur. If we were able to freeze any embryos, we will inform you of the number in a letter.

No fertilisation was observed, so why am I still getting an embryo transfer?

If we only saw one pronucleus in the egg, we may still consider the egg fertilised, because the appearance or disappearance of the pronuclei may have been asynchronous. We may have missed the second pronucleus in our evaluation, which is only a snapshot of a particular moment in time. If the quality of the resulting embryo is sufficient and if no other embryos are available, it can be transferred.Why were more embryos transferred than the number of observed fertilised eggs? Again these are 1PN embryos that are considered as 2PN due to the possible asynchronous appearance or disappearance of the pronuclei.

There were no grade 1, 2 or 3 embryos, so why am I still getting an embryo transfer?

If more than half the embryonic cells are multinucleated, the embryos may be transferred if no other embryos are available.

Why were more embryos used for the embryo biopsy than the number of observed fertilised eggs?

Very exceptionally, a 1PN embryo of sufficient quality can be included in the embryo biopsy. We consider the egg fertilised because we may have missed the second pronucleus in the evaluation (non-synchronised appearance or disappearance of the pronuclei). The genetic diagnosis will prove whether the egg was indeed normally fertilised.