Twins are divided into monozygotic and dizygotic twins, the former being formed by the division of a fertilized egg; the latter being formed by the fertilization of two separate eggs. 1. Monozygotic twins: Twin pregnancies formed by the division of a fertilized egg, which has the same genes. The reason for its formation is not clear, and it is currently believed that it is not affected by race, genetics, age, or number of births. 2. Duplex twin pregnancy: accounts for the majority of twin pregnancies, which are formed by the fertilization of two eggs separately, so their genes are not exactly the same. Its formation may be related to the application of ovulation stimulating drugs, intrauterine transfer of multiple embryos and genetic factors. In case of twin pregnancies, care during pregnancy and delivery should be strengthened, etc. If there is any discomfort, prompt medical attention is recommended.