After fertilization, the zygote, a single cell containing genetic material from both parents, undergoes a series of rapid cell divisions called cleavage. These divisions increase the number of cells without significantly increasing the overall size of the developing embryo. As cleavage progresses, the zygote transforms into a solid ball of cells known as a morula.
Further cell division and rearrangement lead to the formation of a blastocyst, a hollow sphere of cells with two distinct parts: the inner cell mass (which will eventually develop into the embryo) and the outer layer called the trophoblast (which will contribute to the placenta). The blastocyst then implants into the uterine lining, marking the beginning of pregnancy.