From conception to 2 weeks, the zygote is in a period of rapid cell development.By 6 weeks, the embryo's body organs begin to form and function.By 9 weeks, the fetus is recognizably human.Identical twins (monozygotic twins) develop from a single fertilized egg that splits into two; fraternal twins (dizygotic twins) develop from separate fertilized eggs.Teratogens are potentially harmful agents that can pass through the placental screen and interfere with normal development, as happens with fetal alcohol syndrome.