Blastocyst
With in vitro fertilization,  the fertilized egg gets transferred to the uterus. When this is done it is two to three days after fertilization. It is difficult to predict which eggs will develop best. For this reason, several are usually transferred to the uterus. Often this leads to multiple pregnancies (multiple eggs developing into embryos). When a blastocyst is transferred, five to six days after fertilization, this problem does not occur. It is also easier to transfer a blastocyst and attach it to the wall of the uterus.