During embryo development, those less capable of survival do not continue to mature past Day 3 which is the day embryos are traditionally transferred to the uterus.
Survival past Day 3 or 4 after egg collection is important because during the first few days, the embryo relies on the mother’s egg for all its nutrients. To survive past this period, the embryo must activate its own genes and not all embryos are successful in doing this.
Embryos cultured in the laboratory up to Day 5 are called blastocysts. When compared to Day 3 embryos, blastocysts are more highly developed, healthier, stronger, and have a much greater chance of implanting successfully.