Embryo development stage

The blastocyst

What is the blastocyst?

Many couples want to have as many information as possible about their fertility treatment.

One of the most specialized aspects of IVF is the blastocyst.

The blastocyst is a medical term that describes a specific stage of fetal development that is reached after 5 days of culture in the laboratory.

This is a subject of great importance, but also of considerable complexity.

At this stage, the embryo consists of 50-200 cells, which form two groups: the trophoblast (from which the placenta forms) and the fetal pole (from which the fetus forms).


Why is the blastocyst stage important?

  • Only embryos of certain quality manage to reach the blastocyst stage (the rest of the embryos stop developing and die before reaching this stage)
  • Blastocysts are more likely to form a pregnancy compared to embryos in earlier stages of development
  • The lining of the womb is considered to be more embryo-friendly on the 5th day of embryo development (blastocyst stage) than the 2nd or 3rd day of development and this increases the chance of pregnancy after embryo transfer
  • Blastocysts are easier to check with pre-implantation genetic testing


What else do I need to know about blastocysts?

  • Not all embryos are capable of reaching the blastocyst stage

One of the hardest decisions in an IVF cycle is when to do the embryo transfer.

It is a fact that some embryos stop growing before the blastocyst stage. If this happens to all available embryos produced through IVF, then there will be no embryo left for transfer into the womb.

On average (and depending on the number of available embryos and the woman’s age), only 1 out of 2 or 1 out of 3 embryos will reach the blastocyst stage.

Therefore, the decision on whether to continue embryo culture up to the blastocyst stage or not depends on the number and quality of the available embryos on day 3 of development.

If there are many embryos available on day 2 or 3, then they are usually allowed to continue developing, hoping that at least some of them will reach the blastocyst stage.

If day 2 or 3 embryos are few in number, they are usually transferred to the womb immediately.

In other words, allowing embryos to continue growing in the culture is some form of a quality test – only the best embryos grow on to become blastocysts, while the rest stop growing and remove themselves from the pool of available embryos for transfer.


  • A logical question here would be the following:

‘Since reaching the blastocyst stage is a quality test for embryos, why aren’t all embryos cultured up to this stage? And if none of the embryos reaches this stage at the end, could this mean that no embryo was good enough to begin with?’

The answer to this question is a bit complicated:

The embryos are cultured in special incubators that aim to replicate the environment of the tubes and the womb.

But despite all the advances in science, embryo culture in the lab is not exactly the same as embryo development inside the female body.

Therefore, it is possible that some embryos that would develop normally if placed in the womb will not manage to develop normally inside an incubator.

In conclusion, aiming for blastocyst is not the best choice for all embryos.


  • Reaching blastocyst stage is by no means a guarantee that the embryo is of excellent quality and completely healthy

In some occasions, babies derived from embryos that had reached blastocyst stage in the lab have been shown to have chromosomal abnormalities.

For example, a baby with Down’s syndrome conceived naturally started at some point as an embryo which developed normally through all its stages, including the blastocyst stage. Something similar can also happen with an IVF-derived embryo. This too may be genetically abnormal, even if it looks perfect at the blastocyst stage.

Therefore, a blastocyst is simply an advanced stage in the development of an embryo that gives more hope for a pregnancy and nothing more.