Process of growth, differentiation, development, and maturation between fertilization and birth. The fertilized egg immediately begins dividing in the process of cleavage; passes through the stages of morula, blastocyst, gastrula; the three primary germ layers are laid down; and differentiation of the tissues, organs, and organ systems of the body occurs. By 14 weeks all the major organs and systems of the body have been formed; from then on there is further growth and maturation, so that at birth the average fetus is about 20 inches (50 centimeters) long and weighs between 7 and 8 pounds (approx. 3.2-3.6 kilograms).