How does the process of growth and development of a new organism begin?

Let’s consider everything on the example of a person. The process of formation of a new organism and the beginning of its development occurs some time after the fertilization of a female egg by a male sperm. Then a zygote is formed, which begins to actively divide. This is not a new organism yet. The embryo that forms from the zygote begins to develop. This has been going on for several months. Further, the embryo becomes a fetus. After birth, the body continues to grow until about 23 years of age. The body develops all the time until death.