In most clinics dealing with infertility treatment with the use of IVF technique, embryos develop in laboratory settings up to day 2 and 3, i.e. to the moment when they reach 6-8 cell stage. At this time, the best embryos are transferred to the uterus and the remaining ones are usually frozen.
There is a possibility to extend the development of embryos up to the last stage which can be achieved outside the woman’s body. This is the so-called blastocyst culture. Embryos develop in laboratory settings in the media specially designed for this purpose up to the stage of a blastocyst which consists of over 80 cells. Embryos in the blastocyst stage are given to the uterus on day 4 or 5 from ovum pick-up.
- blastocyst stage embryos have higher implantation rate (fetal heart beat for each transferred embryo) when blastocysts are introduced than in the case of transferring 6-8 cell embryos (50.5% vs. 30.1%, P<0.01),
- with the same number of transferred embryos, the transfer of blastocysts gives significantly higher rate of live births (29% – day 3, up to 36% – blastocysts),
- it gives the possibility to select better quality embryos for transfer,
- it allows for better synchronization between the embryo and endometrium at the time of transfer,
- it allows to perform preimplantation genetic diagnosis (PGD) where there are indications for it.
The extended embryo culture gives the chance for obtaining embryo(s) in the blastocyst stage; however, it does not warrant reaching this stage.
- lack of embryos for transfer on day 5 or even on day 6,
- typically lower number of embryos for freezing than in the case of culture up to day 2 or 3,
- some study results demonstrated that while maintaining a high implantation rate, a higher rate of monozygotic twin birth was also observed.