Blastocyst Culture: Day 5 Embryo Transfer Explained

In short: Day 5 transfer means keeping the embryos in the laboratory for 5–6 days until they reach the blastocyst stage, which allows the strongest embryos to be chosen. This raises the chance of implantation and lowers the risk of multiple pregnancy. It suits couples with a good number of embryos or previous failed attempts, but not all embryos reach this stage.

During an IVF cycle, a fertilized egg develops through several stages of division in the laboratory before it is transferred into the uterus, where this natural process of growth continues.

Fertilization usually takes place through conventional in vitro fertilization (IVF) or through intracytoplasmic sperm injection (ICSI). Once fertilized, the egg is carefully monitored as it begins to divide.

By the third day after fertilization, the fertilized egg typically reaches the 4–8 cell stage and is now called an embryo. At this point, it is not yet possible to know for certain whether a particular embryo will continue dividing normally and successfully implant in the endometrium (the inner lining of the uterus). After day 3, the embryo’s cells continue multiplying and its outer shell gradually thins, which supports implantation — a process that normally happens inside the uterus. In certain situations, however, the embryo is not transferred at this early stage. Instead, it is kept in the laboratory under close embryology supervision so its development can be observed further. The embryo is only transferred once it reaches the blastocyst stage, as blastocyst-stage embryos are generally considered to have the best potential for implantation.

How do we decide whether to transfer an embryo on day 2–3 (the 4–8 cell stage) or wait until day 5–6 (the blastocyst stage)?

• The number of previous IVF attempts (blastocyst-stage transfer is now commonly used as a first choice, and is also helpful after repeated unsuccessful cycles)
• The number of embryos available (extending culture to blastocyst stage is usually more suitable when there are more embryos to observe)
• The quality of the embryos (only embryos of good quality on day 3 are generally suitable candidates to continue growing to the blastocyst stage)

What are the benefits of transferring embryos at the blastocyst stage?

• It allows the embryology team to select the embryos that appear strongest for transfer.
• It may improve the likelihood of implantation and pregnancy.
• It can help reduce the risk of multiple pregnancy, since typically fewer embryos need to be transferred.
• It allows time for preimplantation genetic testing (PGT) on day 5–6, when needed, to help avoid transferring embryos with known abnormalities.
• It shortens the waiting period between transfer and the pregnancy test.

What are the possible drawbacks?

Not every embryo continues dividing successfully in the laboratory. This can happen because of factors related to the embryo itself (such as weaker quality), or simply because laboratory culture conditions differ from the natural uterine environment. In some cases, this means a cycle may end without an embryo reaching transfer — an outcome that can understandably be very difficult for couples to hear. Because of this, the decision to extend culture to the blastocyst stage is made carefully, in consultation between the treating doctor and the embryology team, and should always be discussed openly with the couple beforehand so expectations are clear.

Videos Showing Blastocyst Culture

If you have questions about blastocyst culture or would like to discuss whether extended embryo culture may be a suitable option in your treatment plan, the team at

Genetic testing of embryos before transfer is covered in a separate article on preimplantation genetic diagnosis.

Dr Najeeb Layyous’s clinic is here to help guide you through the process.

Dr Najeeb Layyous F.R.C.O.G

Consultant Obstetrician, Gynecologist and Infertility Specialist