There are two mechanisms for the formation of twins; dizygotic twins are two fetuses formed independently from two different fertilized eggs, while monozygotic twins are formed from the division of a single fertilized egg. 1. Dizygotic twins: As the mother ovulates two or more eggs at a time, the two eggs are fertilized separately to develop into two fetuses. These twins have two separate sets of placental circulation, amniotic sacs and chorionic villi. The genes of the two fetuses are genetically different, and the sexes and blood types may not be the same. 2. Monozygotic Twins: One egg splits after fertilization to form two genetically identical fetuses. Monozygotic twin fetuses can be subdivided into three types: double-goat double-velvet, double-goat single-velvet, and single-goat single-velvet. (1) Double amniotic twin: the division occurs before the fertilized egg enters the uterine cavity, usually within 72 hours after fertilization. At this time, the two fertilized eggs obtained from the division are deposited separately, and the two fetuses have separate amniotic sacs and chorionic villi. (2) Double amniotic monochorionic: If the division occurs within 72 hours to 8 days after fertilization, the chorionic villus is already formed at the time of division, and the two fetuses share a common chorionic villus, but in two amniotic sacs. (3) Monoamniotic monochorionic: division occurs within 9 to 13 days of fertilization, the chorionic villus and amniotic sac are already formed, and the two fetuses grow in one chorionic villus and one amniotic sac. After 13 days of fertilization, some of the embryonic structures have formed and the embryo cannot fully split into two individuals, which results in the more dangerous conjoined twins. The risk of a twin pregnancy is higher than that of an ordinary pregnancy, and pregnant women with twins must seek prompt medical attention for any discomfort to avoid danger.