In an IVF cycle, a blastocyst forms in a culture system in a laboratory. ... The embryo divides and multiplies its cells over 5 to 6 days to become a blastocyst. Embryos that survive to this stage of development have a high implantation potential once transferred into the uterine cavity.
By letting nature tell us which embryos are best, it is possible to obtain very high success rates by transferring only a few embryos. We routinely transfer only two blastocyst embryos and, in patients who develop healthy blastocysts, have an ongoing pregnancy rate of greater than 50% per transfer.