Blastocyst Culture


  • A blastocyst is an embryo that has developed in the laboratory for five days after insemination. Blastocysts have a large, central, fluid-filled cavity and two distinct cell types.

    The cells in the centre of the blastocyst are called the inner cell mass and will give rise to the fetus following implantation. The single layer of cells on the outside of the blastocyst forms the trophectoderm and will give rise to the placenta.


  • The human body is the best incubator for human embryos but if there are several embryos to choose from on day 3, we know that not all will survive to day 5 whether they are in the laboratory or the womb. Keeping the embryos in the laboratory for these two extra days eliminates the possibility of transferring a (healthy-looking) embryo on day 3 which would naturally perish before day 5. Therefore it gives a better chance of transferring an embryo which will lead to a successful pregnancy and birth.
  • The decision to choose a day 3 or day 5 embryo transfers will depend on the circumstances at the time. The embryologist will fully discuss this with the patient. We believe it is correct to proceed to day 5 wherever possible and do not believe that finance should play a part in this decision.


  • Blastocyst transfer is not suitable for everyone. It is a laboratory selection process to identify the best embryo. In patients with only 1 or 2 embryos the embryos have already selected themselves for transfer and there is no significant advantage to leaving them outside of the uterus for longer.
  • Not all embryos are happy in the laboratory environment and for this reason you may benefit from a transfer on Day 2 or 3. If your embryos’ development is a little bit slow or if it is already clear which embryo is the strongest for transfer, the embryology team will recommend a Day 2 or Day 3 transfer. It is important not to feel disheartened as there is still a good chance of a successful pregnancy following a transfer on Day 2 or Day 3.