Development by Week
Development begins at fertilization when the sperm penetrates an oocyte and forms a zygote. There is a lot happening each week of development and the process is split into three stages; the germinal stage (weeks 1-2), the embryonic period (weeks 3-8) and the fetal period (weeks 9-birth).
Week 1
Human development begins at fertilization when the sperm and egg unite. This marks the beginning of a new unique individual.
Week 2
During the second week the blastocyst implants in the uterine wall. During this process, morphologic changes in the embryoblast form the bilaminar disc that gives rise to the germ layers that form all of the tissues and organs of the embryo.
Week 3
The third week is characterized by rapid development of the embryo; the primitive streak appears, the notochord starts developing, and the three germ layers differentiate.
Week 4
By the end of the fourth week, neurulation is complete and organogenesis is well on its way.