Identical twins originate from one particular egg that's been fertilized by one sperm. For unknown reasons, the fertilized egg splits into 2 or more embryos during the first stage of development. Some identical twins share the same placenta. However, they generally grow within separate amniotic sacs inside the uterus. In rare cases, identical twins share one amniotic sac (monoamniotic). Most types of childhood arthritis are called juvenile idiopathic arthritis (JIA). JIA causes one or more joint pains and inflammation for at least six weeks.