Pregnancy and parturition
The period of time from fertilization to birth (usually 9 months) is divided into trimesters, each about three months long. During pregnancy the zygote undergoes 40 to 44 rounds of mitosis, producing an infant containing trillions of specialized cells organized into tissues and organs.
The First Trimester
The three embryonic tissue layers form. Cellular differentiation begins to form organs during the third week. After one month the embryo is 5 mm long and composed mostly of paired somite segments.