Cultivation to blastocyst — why do we sometimes wait until the 5th day before transfer?

What is a blastocyst?

Blastocyst is a stage of embryo development that occurs around the 5th day after fertilization. By this time, the embryo has gone through several phases of division (2, 4, 8 cells, etc.), and on the 5th day it begins to differentiate - two main parts are formed:
- Inner cell mass — from which the baby will develop
- Trophectoderm — the cells that will build the placenta.

This is the moment when the embryo naturally leaves the fallopian tube and reaches the uterus.

Benefits of Cultivation to a Blastocyst

Better selection

Only embryos with the best implantation potential manage to reach the blastocyst stage. So we can choose the strongest embryos for transfer or freezing.

Higher transfer success rate

Numerous studies have shown that blastocyst transfer results in a higher chance of implantation compared to embryos on the 2nd or 3rd day.

Better compliance with physiology

In the natural cycle, the embryo reaches the uterus on about the 5th day — the transfer of a blastocyst follows this natural chronology.

More accurate freeze planning and genetic testing

Blastocysts are more stable for vitrification and are more suitable for performing PGT (genetic examination before implantation).

Are there risks when cultivating until the 5th day?

Not every embryo reaches this stage. In couples with a small number of embryos, it is possible that all of them stop developing before day 5. Therefore, the decision is made individually, according to the specific case and in consultation between an embryologist and an obstetrician.

When is an earlier transfer chosen anyway (day 3)?

• With a limited number of embryos;
• With a history of unsuccessful cultivation to a blastocyst;
• For medical or logistical reasons (e.g. age, egg quality, past failed cycles).

Conclusion

Cultivation to a blastocyst is a powerful tool in modern in vitro medicine. It allows better selection and synchronization with the physiology of the female organism, which often leads to higher results. At the same time, it is not a universal solution, and the approach should always be individual.
In our clinic, we make these decisions with care, based on experience and with clear communication with patients — because each embryo is important, and each path to pregnancy is different.

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