Blastocyst
A blastocyst is an embryo that has developed for about five to six days after fertilisation. Transferring at this stage can help the lab choose the embryo most likely to implant.
A blastocyst is an embryo that has grown in the lab for about five to six days after fertilisation, by which point it has many cells and a clear structure.
Why day five matters
Growing embryos to the blastocyst stage helps the lab see which are developing well, which can improve the chance that a single transferred embryo implants.
Sources and further reading
- ESHRE/ALPHA: Embryo morphology consensus (2025). Embryo developmental stages.
Source links checked on 7 September 2026. This source check is separate from medical review.
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