In which period does the blastocyst become an embryo?

The transition of the blastocyst to an embryo occurs during the embryonic period. This stage is typically defined as beginning after the first two weeks post-fertilization, where the blastocyst, which has formed from the fertilized egg, implants into the uterine wall. Once implantation occurs, the blastocyst begins to differentiate into various cell layers which will ultimately develop into the major systems and structures of the embryo.

This period is critical for organogenesis, the formation of organs, and takes place from roughly the 3rd week to the 8th week of gestation. During this time, all major organ systems start to develop, and the basic body plan is established, marking the transition from a single-cell fertilized egg to a multi-cellular embryonic structure.

Other periods such as the pre-embryonic period refers to the first two weeks where implantation occurs, the fetal period follows after the embryonic stage and focuses on growth and maturation of the already formed tissues, and the postnatal period refers to the time after birth. Thus, the identification of the embryonic period is essential in understanding when the blastocyst fully transitions into the embryo stage.