The second or third day of embryo development has proven to be a good time for embryo transfer in IVF or ICSI treatment. Due to medical progress, it has become possible to cultivate embryos for five days until the blastocyst stage.
The later replacement of embryos on day five makes it possible to observe the development of the embryos for longer. Due to the longer time between ovum collection and embryo transfer, the body has more time to recover from the hormonal treatment. A blastocyst thereby often has a higher probability of implanting and thereby leading to a pregnancy.
Blastocyst cultivation is most frequently carried out when many fertilized ova are available. If an embryo reaches the blastocyst stage, then the protective shell (also known as the zona pellucida) that surrounds it is very thin. The assisted hatching procedure is therefore not carried out on embryos in the blastocyst stage. In the case of cryopreserved ova, a blastocyst cultivated is also possible but is rarely carried out.
The use of sequential media has made extended culture and transfer of blastocysts feasible for human IVF. Embryo transfer on day five has been claimed to result in higher implantation rates than transfer on day 3, based on retrospective comparative studies. This is not supported convincingly, however, in randomized controlled trials published to date. Blastocyst culture imposes additional requirements in terms of personnel, equipment, education and cost. It is associated with a greater incidence of monozygotic twinning and cycle cancellation rate than in the case of day three culture. For day five transfer to replace day three transfer, a convincing comparison between the two methods should demonstrate blastocyst transfer’s superiority. There is still a need for adequately designed randomized controlled trials to compare day 3 with day five transfer which will also address the effectiveness of a single blastocyst replacement in reducing the incidence of multiple pregnancies and the value of blastocyst cryopreservation.