Following fertilization, the fertilized egg begins as a 2 pronuclear (2PN) preimplantation embryo, called a zygote, a single cell containing genetic material from both the mother and the father.
The zygote undergoes divisions called cleavages. It divides into 2 cells and from there eventually into 6-8 cells (cleavage stage).
At this point the embryo can be transferred into the uterus, but another option is to allow further growth to the blastocyst stage, called blastocyst culture.
The blastocyst represents the final maturation stage of the fertilized egg prior to implantation.
During culture, the embryos are grown an extra 2-3 days prior to transfer (day 5-day 6 following retrieval). Only the healthy embryos continue to develop to the blastocyst stage. Extended embryo culture can allow for fewer transferred preembryos by transfering only the most healthy. This decreases the risk of multiple pregnancy.
