What major changes occur during the pre-embryonic stage of human development?

The pre-embryonic stage of human development encompasses the first two weeks following conception. This period is primarily characterized by rapid cell division and the initial stages of cell differentiation.1

Key developmental changes during this stage include:

• Cleavage: Following fertilization, the zygote undergoes a series of mitotic divisions known as cleavage. These divisions produce smaller cells called blastomeres, which do not increase the total mass of the organism.23
• Morula Formation: As the cluster of cells travels through the uterine tube toward the uterus, it forms a solid, mulberry-like mass of approximately 12 to 60 or more cells called a morula.24
• Blastocyst Development: Upon reaching the uterus, the morula develops into a hollow, fluid-filled structure known as a blastocyst. This structure consists of an inner cell mass, which will develop into the embryo, and an outer layer called the trophoblast, which contributes to the formation of the placenta and fetal membranes.24
• Implantation: The pre-embryonic stage concludes with the implantation of the blastocyst into the uterine lining.45