Here you shall find a clarification of the different stages of fetal growth, from the very first stage, called zygote, to the last phase, in which it is known as fetus. It will be called this way up until childbirth.
Embryo biopsy is a step in the process of PGD which consists of removing one or two cells from the embryo without compromising its normal growth so that an assessment and a genetic study of the embryo can be conducted. The biopsy can be performed before or after fertilisation, the former by biopsying the polar body, and the latter by biopsying a blastomere from the embryo on day 3 or on day 5.
Blastocyst is the stage in which embryo implantation within the uterus occurs naturally. To reach this stage, the embryo is cultured up to day 5-6 by means of a sequential culture system.
Embryos can be classified in 4 categories –from A to D– ranging from best quality to least developed embryos. This classification is mainly visual and rather subjective, but there are also several studies to identify their metabolic activity.