What process must occur after fertilization in all stages for development to coutinue?

After fertilization, the zygote undergoes a series of cell divisions known as cleavage, leading to the formation of a multicellular structure called a blastocyst. This stage is crucial as the blastocyst must implant into the uterine lining for further development. Following implantation, the embryo begins to differentiate into various cell types through the process of gastrulation, establishing the fundamental body plan. Hormonal changes also support the pregnancy, ensuring the environment is suitable for continued development.