Biopsying the embryo at Day 5 or 6 of growth allows us to determine whether the embryo has a normal number of chromosomes. The chance that the embryo does not have normal chromosomes increases as the woman ages. An embryo that forms a blastocyst (has more than 100 cells inside on Day 5-6) and that is genetically normal has a much higher chance of leading to a normal pregnancy than an untested embryo that is 2 or 3 days old. The quality of the embryology laboratory must be very high in order to grow embryos until Day 5-6 and perform this advanced genetic testing. It is expected that some embryos will stop growing between Day 2 and Day 6. This is either caused by poor embryo quality and/or poor environmental factors in the embryo laboratory.