By the morning of day two, embryos should divide to four cells, by the morning of day three, they should reach the eight-cell stage.
Two days later, the healthiest embryos will reach the blastocyst stage. Blastocyst consists of a hundred or more cells surrounding a tiny cavity at the center of the blastocyst.
Most embryo transfers are done 2 to 3 days after the egg retrieval. Transferring an embryo early in the first couple of days can minimize the out of body exposures that can affect the embryo.
Day 5 (blastocyst) transfer can be used to minimize the risk of a multiple pregnancy by transferring only one or two of the very best embryos.
The blastocyst transfer is only used for selected patients, since the extended culture may not be ideal for some embryos that may have caused pregnancy if they had been transferred on day two or three.
