This is a procedure that allows a longer time for the embryo to develop in the laboratory (five days instead of two to three). This enables the embryo to reach the Blastocyst stage, which is the natural embryonic stage for implantation in the uterus.

Many couples undergoing In Vitro Fertilization accept the risk of multiple gestation as an integral part of the treatment process. They want to maximize their chances of achieving a pregnancy and will transfer three or more embryos to achieve their goal. After all, the chance of taking home a baby is not 100%, and the cost of an IVF cycle can be quite significant. Sometimes, the financial resources of a particular couple may only allow them to undergo an IVF cycle once in their lifetime. The end result of transferring 3 or more embryos has been a significant increase in the rate of triplet and higher order multiple pregnancies. There are enormous medical, social, and financial consequences of this increase in multiple birth rate.

One solution to this problem may be to culture the embryos for 5 or 6 days, to the blastocyst stage. Embryos developing to the blastocyst stage (blastocysts), have a higher implantation rate than embryos grown only three days, and are more likely to succeed in initiating a pregnancy. As result, only two blastocysts need to be transferred to have the same pregnancy rate usually seen when 3 or more embryos that have been grown for only 2 or 3 days are transferred into the uterus. In other words, fewer embryos are needed to achieve the same or higher pregnancy rate, resulting in a lower incidence of multiple gestation.