In the case of monozygotic twins, the fertilized egg may divide after the 13th day after fertilization at the latest, which means that it divides to form twins about two weeks after fertilization at the latest. Twin pregnancies formed by the division of a single fertilized egg are called monozygotic twins and can be classified into four types due to the timing of the division of the fertilized egg that occurs during the early stages of development. 1. Double chorionic villus and double amniotic sac monozygotic twins, the division occurs at the mulberry stage, equivalent to three days after fertilization, two separate embryos and two amniotic sacs are formed, which accounts for about 30% of monozygotic twins. 2. Monochorionic double amniotic sac monozygotic twins, the division usually occurs on the 4th to 8th day after fertilization, the embryo development is in the blastocyst stage, this type accounts for about 68% of monozygotic twins. 3. monochorionic monoamniotic sac monozygotic twins, the fertilized egg usually splits on the 9th to 13th day after fertilization, relatively rare, accounting for about 1% to 2% of monozygotic twins. 4. conjoined twins, the fertilized egg splits after the 13th day of fertilization, that is, about two weeks, because at this time the primordial embryonic disk has already been formed, the body is not able to completely split into two, it will form different forms of conjoined children, relatively rare. It is recommended to go to the hospital for timely examination after pregnancy to clarify intrauterine pregnancy.