A blastocyst is an embryo that is five days old and has already undergone a certain degree of development. Those cells that will form the embryo itself can already be distinguished, as can those cells that will be responsible for the placenta.
- for couples with several good embryos on day 3. The biologist can monitor the embryos 2 days longer. However, not every embryo develops to the blastocyst stage. Some will be blocked in their development before reaching the blastocyst stage. There will thus be fewer to freeze, if that becomes appropriate.
- for couples suspected of having implantation problems. A blastocyst will implant itself more quickly after embryo transfer than an embryo on day three (this will still have to ‘survive’ for 2-3 days in the uterus). If the environment in the uterus is less favourable to the development of an embryo, a blastocyst transfer can be useful.