In natural conception, embryo travels to the uterus on 5th or 6th day after fertilization and implants itself.
In IVF, embryos are put into uterus on 3rd day of fertilization. But there have been recent advances in lab techniques and availability of special media to allow embryos to grow till 4th to 6th day and evolve into a blastocyst.
This confirms that embryo is of very good quality and is capable of implanting itself and grow further.This decreases the chances of multifoeatal pregnancy since it requires lesser number of embryos to be transferred for a high fertilization rate.